High-speed light sensing apparatus ii

ABSTRACT

An optical apparatus including a semiconductor substrate; a first light absorption region supported by the semiconductor substrate, the first light absorption region configured to absorb photons and to generate photo-carriers from the absorbed photons; one or more first switches controlled by a first control signal, the one or more first switches configured to collect at least a portion of the photo-carriers based on the first control signal; and one or more second switches controlled by a second control signal, the one or more second switches configured to collect at least a portion of the photo-carriers based on the second control signal. The one or more first switches include a first trench located between the first p-doped region and the first n-doped region. The one or more second switches include a second trench located between the second p-doped region and the second n-doped region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 15/338,660 filed on Oct. 31, 2016, entitled “HIGH-SPEED LIGHTSENSING APPARATUS,” and claims the benefit of U.S. Provisional PatentApplication No. 62/465,139, filed on Feb. 28, 2017; U.S. ProvisionalPatent Application No. 62/479,322, filed on Mar. 31, 2017; U.S.Provisional Patent Application No. 62/504,531, filed on May 10, 2017;U.S. Provisional Patent Application No. 62/485,003, filed on Apr. 13,2017; U.S. Provisional Patent Application No. 62/511,977, filed on May27, 2017; U.S. Provisional Patent Application No. 62/534,179, filed onJul. 18, 2017; U.S. Provisional Patent Application No. 62/561,266, filedon Sep. 21, 2017; U.S. Provisional Patent Application No. 62/613,054,filed on Jan. 3, 2018; and U.S. Provisional Patent Application No.62/617,317, filed on Jan. 15, 2018, all of which are incorporated byreference in their entirety.

BACKGROUND

This specification relates to detecting light using a photodetector.

Light propagates in free space or an optical medium is coupled to aphotodetector that converts an optical signal to an electrical signalfor processing.

SUMMARY

According to one innovative aspect of the subject matter described inthis specification, light reflected from a three-dimensional object maybe detected by photodetectors of an imaging system. The photodetectorsconvert the detected light into electrical charges. Each photodetectormay include two groups of switches that collect the electrical charges.The collection of the electrical charges by the two groups of switchesmay be altered over time, such that the imaging system may determinephase information of the sensed light. The imaging system may use thephase information to analyze characteristics associated with thethree-dimensional object including depth information or a materialcomposition. The imaging system may also use the phase information toanalyze characteristics associated with eye-tracking, gesturerecognition, 3-dimensional model scanning/video recording, motiontracking, and/or augmented/virtual reality applications.

In general, one innovative aspect of the subject matter described inthis specification can be embodied in an optical apparatus that includesa semiconductor substrate; a germanium-silicon layer coupled to thesemiconductor substrate, the germanium-silicon layer including aphotodetector region configured to absorb photons and to generatephoto-carriers from the absorbed photons; one or more first switchescontrolled by a first control signal, the one or more first switchesconfigured to collect at least a portion of the photo-carriers based onthe first control signal; and one or more second switches controlled bya second control signal, the one or more second switches configured tocollect at least a portion of the photo-carriers based on the secondcontrol signal, where the second control signal is different from thefirst control signal. The one or more first switches include a firstp-doped region in the germanium-silicon layer, where the first p-dopedregion is controlled by the first control signal; and a first n-dopedregion in the germanium-silicon layer, where the first n-doped region iscoupled to a first readout integrated circuit. The one or more secondswitches include a second p-doped region in the germanium-silicon layer,where the second p-doped region is controlled by the second controlsignal; and a second n-doped region in the germanium-silicon layer,where the second n-doped region is coupled to a second readoutintegrated circuit.

This and other implementations can each optionally include one or moreof the following features. The germanium-silicon layer may include athird n-doped region and a fourth n-doped region, where at least aportion of the first p-doped region may be formed in the third n-dopedregion, and where at least a portion of the second p-doped region may beformed in the fourth n-doped region. The germanium-silicon layer mayinclude a third n-doped region, where at least a portion of the firstp-doped region and a portion of the second p-doped region may be formedin the third n-doped region. The semiconductor substrate may include athird p-doped region and one or more n-doped regions, where thegermanium-silicon layer may be arranged over the third p-doped region,and where the third p-doped region may be electrically shorted with theone or more n-doped regions.

The first control signal may be a fixed bias voltage, and the secondcontrol signal may be a variable bias voltage that is biased over thefixed voltage of the first control signal. The photons absorbed by thegermanium-silicon layer may be reflected from a surface of athree-dimensional target, and the portion of the photo-carrierscollected by the one or more first switches and the portion of thephoto-carriers collected by the one or more second switches may beutilized by a time-of-flight system to analyze depth information or amaterial composition of the three-dimensional target.

Another innovative aspect of the subject matter described in thisspecification can be embodied in an optical apparatus including asemiconductor substrate; an absorption layer coupled to thesemiconductor substrate, the absorption layer including a photodetectorregion configured to absorb photons and to generate photo-carriers fromthe absorbed photons; one or more first switches controlled by a firstcontrol signal, the one or more first switches configured to collect atleast a portion of the photo-carriers based on the first control signal;and one or more second switches controlled by a second control signal,the one or more second switches configured to collect at least a portionof the photo-carriers based on the second control signal, where thesecond control signal is different from the first control signal. Theone or more first switches include a first p-doped region in thesemiconductor substrate, where the first p-doped region is controlled bythe first control signal; and a first n-doped region in thesemiconductor substrate, where the first n-doped region is coupled to afirst readout integrated circuit. The one or more second switchesinclude a second p-doped region in the semiconductor substrate, wherethe second p-doped region is controlled by the second control signal;and a second n-doped region in the semiconductor substrate, wherein thesecond n-doped region is coupled to a second readout integrated circuit.

This and other implementations can each optionally include one or moreof the following features. The semiconductor substrate may include athird n-doped region and a fourth n-doped region, where at least aportion of the first p-doped region may be formed in the third n-dopedregion, and where at least a portion of the second p-doped region may beformed in the fourth n-doped region. The semiconductor substrate mayinclude a third n-doped region, where at least a portion of the firstp-doped region and a portion of the second p-doped region may be formedin the third n-doped region. The semiconductor substrate may include oneor more p-well regions.

The first control signal may be a fixed bias voltage, where the secondcontrol signal may be a variable bias voltage that is biased over thefixed voltage of the first control signal. The photons absorbed by theabsorption layer may be reflected from a surface of a three-dimensionaltarget, where the portion of the photo-carriers collected by the one ormore first switches and the portion of the photo-carriers collected bythe one or more second switches may be utilized by a time-of-flightsystem to analyze depth information or a material composition of thethree-dimensional target.

Another innovative aspect of the subject matter described in thisspecification can be embodied in an optical apparatus including asemiconductor substrate; an absorption layer coupled to thesemiconductor substrate, the absorption layer including a photodetectorregion configured to absorb photons and to generate photo-carriers fromthe absorbed photons; one or more first switches controlled by a firstcontrol signal, the one or more first switches configured to collect atleast a portion of the photo-carriers based on the first control signal;and one or more second switches controlled by a second control signal,the one or more second switches configured to collect at least a portionof the photo-carriers based on the second control signal, where thesecond control signal is different from the first control signal. Theone or more first switches include multiple first p-doped regions in thesemiconductor substrate, where the multiple first p-doped regions arecontrolled by the first control signal; and multiple first n-dopedregions in the semiconductor substrate, where the multiple first n-dopedregions are coupled to a first readout integrated circuit. The one ormore second switches include multiple second p-doped regions in thesemiconductor substrate, where the multiple second p-doped regions arecontrolled by the second control signal; and multiple second n-dopedregions in the semiconductor substrate, where the multiple secondn-doped regions are coupled to a second readout integrated circuit.

This and other implementations can each optionally include one or moreof the following features. The semiconductor substrate may include athird n-doped region, where at least a portion of the multiple firstp-doped region and a portion of the multiple second p-doped region maybe formed in the third n-doped region. The multiple first p-dopedregions and the multiple second p-doped regions may be arranged in aninterdigitated arrangement along a first plane in the semiconductorsubstrate, where the multiple first n-doped regions and the multiplesecond n-doped regions may be arranged in an interdigitated arrangementalong a second plane in the semiconductor substrate that is differentfrom the first plane. Each p-doped region of the multiple first p-dopedregions may be arranged over a respective n-doped region of the multiplefirst n-doped regions, and each p-doped region of the multiple secondp-doped regions may be arranged over a respective n-doped region of themultiple second n-doped regions. The semiconductor substrate may includeone or more p-well regions.

The first control signal may be a fixed bias voltage, and the secondcontrol signal may be a variable bias voltage that is biased over thefixed voltage of the first control signal. The photons absorbed by theabsorption layer may be reflected from a surface of a three-dimensionaltarget, where the portion of the photo-carriers collected by the one ormore first switches and the portion of the photo-carriers collected bythe one or more second switches may be utilized by a time-of-flightsystem to analyze depth information or a material composition of thethree-dimensional target.

Another innovative aspect of the subject matter described in thisspecification can be embodied in a time-of-flight system that includes alight source; and an image sensor comprising multiple pixels fabricatedon a semiconductor substrate, where each pixel of the pixels includes agermanium-silicon layer coupled to the semiconductor substrate, thegermanium-silicon layer including a photodetector region configured toabsorb photons and to generate photo-carriers from the absorbed photons;one or more first switches controlled by a first control signal, the oneor more first switches configured to collect at least a portion of thephoto-carriers based on the first control signal; and one or more secondswitches controlled by a second control signal, the one or more secondswitches configured to collect at least a portion of the photo-carriersbased on the second control signal, where the second control signal isdifferent from the first control signal.

This and other implementations can each optionally include one or moreof the following features. The light source may be configured to emitoptical pulses having a duty cycle that is less than 50% but maintaininga same amount of energy per optical pulse.

Another innovative aspect of the subject matter described in thisspecification can be embodied in an optical apparatus that includes asemiconductor substrate; a germanium-silicon layer coupled to thesemiconductor substrate, the germanium-silicon layer including aphotodetector region configured to absorb photons and to generatephoto-carriers from the absorbed photons; one or more first switchescontrolled by a first control signal, the one or more first switchesconfigured to collect at least a portion of the photo-carriers based onthe first control signal; and one or more second switches controlled bya second control signal, the one or more second switches configured tocollect at least a portion of the photo-carriers based on the secondcontrol signal, where the second control signal is different from thefirst control signal. The one or more first switches include a firstp-doped region in the germanium-silicon layer, where the first p-dopedregion is controlled by the first control signal; and a first n-dopedregion in the semiconductor substrate, where the first n-doped region iscoupled to a first readout integrated circuit. The one or more secondswitches include a second p-doped region in the germanium-silicon layer,where the second p-doped region is controlled by the second controlsignal; and a second n-doped region in the semiconductor substrate,where the second n-doped region is coupled to a second readoutintegrated circuit.

This and other implementations can each optionally include one or moreof the following features. The germanium-silicon layer may include athird n-doped region and a fourth n-doped region, where at least aportion of the first p-doped region is formed in the third n-dopedregion, and where at least a portion of the second p-doped region may beformed in the fourth n-doped region. The germanium-silicon layer mayinclude a third n-doped region, where at least a portion of the firstp-doped region and a portion of the second p-doped region may be formedin the third n-doped region. The semiconductor substrate may include oneor more p-well regions.

The first control signal may be a fixed bias voltage, where the secondcontrol signal may be a variable bias voltage that is biased over thefixed voltage of the first control signal. The photons absorbed by thegermanium-silicon layer may be reflected from a surface of athree-dimensional target, where the portion of the photo-carrierscollected by the one or more first switches and the portion of thephoto-carriers collected by the one or more second switches may beutilized by a time-of-flight system to analyze depth information or amaterial composition of the three-dimensional target.

Another innovative aspect of the subject matter described in thisspecification can be embodied in an optical apparatus that includes asemiconductor substrate; a first light absorption region supported bythe semiconductor substrate, the first light absorption region includinggermanium and configured to absorb photons and to generatephoto-carriers from the absorbed photons; a first layer supported by atleast a portion of the semiconductor substrate and the first lightabsorption region, the first layer being different from the first lightabsorption region; one or more first switches controlled by a firstcontrol signal, the one or more first switches configured to collect atleast a portion of the photo-carriers based on the first control signal;and one or more second switches controlled by a second control signal,the one or more second switches configured to collect at least a portionof the photo-carriers based on the second control signal, wherein thesecond control signal is different from the first control signal. Theone or more first switches include a first control contact coupled to afirst control region of the first layer, wherein the first controlregion is controlled by the first control signal; and a first readoutcontact coupled to a first readout region of the first layer, whereinthe first readout region is coupled to a first readout integratedcircuit. The one or more second switches include a second controlcontact coupled to a second control region of the first layer, whereinthe second control region is controlled by the second control signal;and a second readout contact coupled to a second readout region of thefirst layer, wherein the second readout region is coupled to a secondreadout integrated circuit.

Embodiments of the optical apparatus can include one or more of thefollowing features. For example, the semiconductor substrate can includea recess, and at least a portion of the first light absorption regioncan be embedded in the recess. The first layer can be a silicon layer ora germanium-silicon layer. The first layer can include a CMOSprocess-compatible material.

The first light absorption region can be formed from germanium orgermanium-silicon.

In some embodiments, the first readout region can include a firstn-doped region and the second readout region can include a secondn-doped region. The first readout region and the second readout regioncan be supported by the first light absorption region, and the firstcontrol region and the second control region can be supported by thefirst light absorption region. The first control region can include afirst p-doped region and the second control region can include a secondp-doped region. The first light absorption region can include a thirdn-doped region located beneath the first control region of the firstlayer and in contact with the first p-doped region; and a fourth n-dopedregion located beneath the second control region of the first layer andin contact with the second p-doped region. The first light absorptionregion can include a third p-doped region; and a fourth p-doped region.

In some embodiments, the first light absorption region can include athird n-doped region located beneath the first control region of thefirst layer; and a fourth n-doped region located beneath the secondcontrol region of the first layer.

In some embodiments, the first readout region and the second readoutregion can be supported by the semiconductor substrate, and the firstcontrol region and the second control region can be supported by thesemiconductor substrate. The first control region can include a firstp-doped region and the second control region can include a secondp-doped region. The semiconductor substrate can include a third n-dopedregion located beneath the first control region of the first layer andin contact with the first p-doped region; and a fourth n-doped regionlocated beneath the second control region of the first layer and incontact with the second p-doped region. The semiconductor substrate caninclude a third p-doped region; and a fourth p-doped region.

In some embodiments, the semiconductor substrate can include a thirdn-doped region located beneath the first control region of the firstlayer; and a fourth n-doped region located beneath the second controlregion of the first layer.

In some embodiments, the one or more first switches can further includea third control contact coupled to a third control region of the firstlayer, wherein the third control region is supported by the first lightabsorption region and controlled by a third control signal; and a fourthcontrol contact coupled to a fourth control region of the first layer,wherein the fourth control region is supported by the first lightabsorption region and controlled by a fourth control signal. The thirdcontrol region can include a third p-doped region and the fourth controlregion can include a fourth p-doped region. The semiconductor substratecan include a third n-doped region located beneath the first controlregion of the first layer and in contact with the first p-doped region;and a fourth n-doped region located beneath the second control region ofthe first layer and in contact with the second p-doped region. The firstlight absorption region can include a fifth n-doped region locatedbeneath the third control region of the first layer and in contact withthe third p-doped region; and a sixth n-doped region located beneath thefourth control region of the first layer and in contact with the fourthp-doped region. The semiconductor substrate can further include: a fifthp-doped region; and a sixth p-doped region.

In some embodiments, the semiconductor substrate can include a thirdn-doped region located beneath the first control region of the firstlayer and in contact with the first p-doped region; and a fourth n-dopedregion located beneath the second control region of the first layer andin contact with the second p-doped region. The first light absorptionregion can include a fifth n-doped region located beneath the thirdcontrol region of the first layer; and a sixth n-doped region locatedbeneath the fourth control region of the first layer.

In some embodiments, the optical apparatus can further include a firstbipolar junction transistor and a second bipolar junction transistor.The first bipolar junction transistor can include a first electronemitter supported by the semiconductor substrate; the first p-dopedregion; and the first n-doped region. The second bipolar junctiontransistor can include a second electron emitter supported by thesemiconductor substrate; the second p-doped region; and the secondn-doped region.

Another innovative aspect of the subject matter described in thisspecification can be embodied in an optical apparatus that includes asemiconductor substrate; a first light absorption region supported bythe semiconductor substrate, the first light absorption regionconfigured to absorb photons and to generate photo-carriers from theabsorbed photons; one or more first switches controlled by a firstcontrol signal, the one or more first switches configured to collect atleast a portion of the photo-carriers based on the first control signal;one or more second switches controlled by a second control signal, theone or more second switches configured to collect at least a portion ofthe photo-carriers based on the second control signal, wherein thesecond control signal is different from the first control signal; and acounter-doped region formed in a first portion of the first lightabsorption region, the counter-doped region including a first dopant andhaving a first net carrier concentration lower than a second net carrierconcentration of a second portion of the first light absorption region.The one or more first switches include a first control contact coupledto a first control region, wherein the first control region iscontrolled by the first control signal; and a first readout contactcoupled to a first readout region, wherein the first readout region iscoupled to a first readout integrated circuit. The one or more secondswitches include a second control contact coupled to a second controlregion, wherein the second control region is controlled by the secondcontrol signal; and a second readout contact coupled to a second readoutregion, wherein the second readout region is coupled to a second readoutintegrated circuit.

Embodiments of the optical apparatus can include one or more of thefollowing features. For example, during operation, the counter-dopedregion can reduce a leakage current flowing between the first controlcontact and the second control contact relative to a comparable opticalapparatus without the counter-doped region.

In some embodiments, the first control region, the first readout region,the second control region, and the second readout region can besupported by the first light absorption region, and the counter-dopedregion includes at least a portion of the first control region, thefirst readout region, the second control region, and the second readoutregion. The first readout region can include a first n-doped region andthe second readout region can include a second n-doped region. The firstcontrol region can include a first p-doped region and the second controlregion can include a second p-doped region. The optical apparatus canfurther include a third n-doped region in contact with the first p-dopedregion; and a fourth n-doped region in contact with the second p-dopedregion, wherein a first lateral separation between the third n-dopedregion and the fourth n-doped region is smaller than a second lateralseparation between the first p-doped region and the second p-dopedregion.

The first light absorption region can include germanium orgermanium-silicon. The first dopant of the counter-doped region can beselected from the group consisting of phosphorous, arsenic, antimony,and fluorine. A doping concentration of the counter-doped region can bebetween 2*10¹³/cm³ and 5*10¹⁴/cm³. A doping concentration of thecounter-doped region can be larger than a defect concentration of thegermanium or the germanium-silicon.

In some embodiments, the optical apparatus can further include a firstreflector supported by the semiconductor substrate. The first reflectorcan be one or more of a metal mirror; a dielectric mirror; and adistributed Bragg reflector. The optical apparatus can further include asecond reflector supported by the semiconductor substrate, wherein thefirst reflector and the second reflector are located on opposite sidesof the first light absorption region. The optical apparatus can furtherinclude a first anti-reflection layer supported by the semiconductorsubstrate, wherein the first reflector and the first anti-reflectionlayer are located on opposite sides of the first light absorptionregion.

In some embodiments, the optical apparatus can further include a lenssupported by the semiconductor substrate. The lens can be integrallyformed on the semiconductor substrate. The optical apparatus can furtherinclude a spacer layer supported by the semiconductor substrate,wherein, in a direction normal to a substrate surface, the spacer layeris arranged between the first light absorption region and the lens. Theoptical apparatus can further include a second anti-reflection layersupported by the semiconductor substrate and arranged between thesemiconductor substrate and the lens. A refractive index of at least aportion of the second anti-reflection layer can be greater than 1.8. Thesecond anti-reflection layer can include a CMOS process-compatiblehigh-k material.

In some embodiments, the optical apparatus can further include a firstlayer supported by at least a portion of the semiconductor substrate andthe first light absorption region, the first layer being different fromthe first light absorption region.

Another innovative aspect of the subject matter described in thisspecification can be embodied in an optical apparatus that include asemiconductor substrate; a first light absorption region supported bythe semiconductor substrate, the first light absorption regionconfigured to absorb photons and to generate photo-carriers from theabsorbed photons; one or more first switches controlled by a firstcontrol signal, the one or more first switches configured to collect atleast a portion of the photo-carriers based on the first control signal;and one or more second switches controlled by a second control signal,the one or more second switches configured to collect at least a portionof the photo-carriers based on the second control signal, wherein thesecond control signal is different from the first control signal. Theone or more first switches include a first p-doped region in the firstlight absorption region, wherein the first p-doped region is controlledby the first control signal and has a first p-dopant concentration; asecond p-doped region in the first light absorption region and incontact with at least a first portion of the first p-doped region,wherein the second p-doped region has a second p-dopant concentrationlower than the first p-dopant concentration; and a first n-doped regionin the first light absorption region, wherein the first n-doped regionis coupled to a first readout integrated circuit and has a firstn-dopant concentration. The one or more second switches include a thirdp-doped region in the first light absorption region, wherein the thirdp-doped region is controlled by the second control signal and has athird p-dopant concentration; a fourth p-doped region in the first lightabsorption region and in contact with at least a first portion of thethird p-doped region, wherein the fourth p-doped region has a fourthp-dopant concentration lower than the third p-dopant concentration; anda second n-doped region in the first light absorption region, whereinthe second n-doped region is coupled to a second readout integratedcircuit and has a second n-dopant concentration.

Embodiments of the optical apparatus can include one or more of thefollowing features. For example, during operation, the second p-dopedregion can reduce a first dark current flowing between the first p-dopedregion and the first n-doped region, and the fourth p-doped region canreduce a second dark current flowing between the third p-doped regionand the second n-doped region relative to a comparable optical apparatuswithout the second and fourth p-doped regions.

In some embodiments, the one or more first switches can further includea third n-doped region in the first light absorption region and incontact with at least a portion of the first n-doped region, wherein thethird n-doped region has a third n-dopant concentration lower than thefirst n-dopant concentration, and the one or more second switches canfurther include a fourth n-doped region in the first light absorptionregion and in contact with at least a portion of the second n-dopedregion, wherein the fourth n-doped region has a fourth n-dopantconcentration lower than the second n-dopant concentration. Duringoperation, the third n-doped region can reduce a first dark currentflowing between the first p-doped region and the first n-doped region,and the fourth n-doped region can reduce a second dark current flowingbetween the third p-doped region and the second n-doped region relativeto a comparable optical apparatus without the third and fourth n-dopedregions.

In some embodiments, the first light absorption region can includegermanium or germanium-silicon. The optical apparatus can furtherinclude a first layer supported by the first light absorption region,the first layer being different from the first light absorption region.The one or more first switches can further include a fifth n-dopedregion in contact with a second portion of the first p-doped region, andthe one or more second switches can further include a sixth n-dopedregion in contact with a second portion the third p-doped region.

Another innovative aspect of the subject matter described in thisspecification can be embodied in an optical apparatus that includes asemiconductor substrate; a first light absorption region supported bythe semiconductor substrate, the first light absorption regionconfigured to absorb photons and to generate photo-carriers from theabsorbed photons; one or more first switches controlled by a firstcontrol signal, the one or more first switches configured to collect atleast a portion of the photo-carriers based on the first control signal;and one or more second switches controlled by a second control signal,the one or more second switches configured to collect at least a portionof the photo-carriers based on the second control signal, wherein thesecond control signal is different from the first control signal. Theone or more first switches include a first p-doped region in the firstlight absorption region, wherein the first p-doped region is controlledby the first control signal and has a first p-dopant concentration; afirst n-doped region in the first light absorption region, wherein thefirst n-doped region is coupled to a first readout integrated circuitand has a first n-dopant concentration; and a first trench locatedbetween the first p-doped region and the first n-doped region. The oneor more second switches include a second p-doped region in the firstlight absorption region, wherein the second p-doped region is controlledby the second control signal and has a second p-dopant concentration; asecond n-doped region in the first light absorption region, wherein thesecond n-doped region is coupled to a second readout integrated circuitand has a second n-dopant concentration; and a second trench locatedbetween the second p-doped region and the second n-doped region.

Embodiments of the optical apparatus can include one or more of thefollowing features. For example, during operation, the first trench canreduce a first dark current flowing between the first p-doped region andthe first n-doped region, and the second trench can reduce a second darkcurrent flowing between the second p-doped region and the second n-dopedregion relative to a comparable optical apparatus without the first andsecond trenches.

In some embodiments, the first light absorption region can includegermanium or germanium-silicon. The one or more first switches canfurther include a third p-doped region in the first light absorptionregion and in contact with at least a first portion of the first p-dopedregion, wherein the third p-doped region has a third p-dopantconcentration lower than the first p-dopant concentration; and a thirdn-doped region in the first light absorption region and in contact withat least a portion of the first n-doped region, wherein the thirdn-doped region has a third n-dopant concentration lower than the firstn-dopant concentration. The one or more second switches can furtherinclude a fourth p-doped region in the first light absorption region andin contact with at least a first portion of the second p-doped region,wherein the fourth p-doped region has a fourth p-dopant concentrationlower than the second p-dopant concentration; and a fourth n-dopedregion in the first light absorption region and in contact with at leasta portion of the second n-doped region, wherein the fourth n-dopedregion has a fourth n-dopant concentration lower than the secondn-dopant concentration. During operation, the third n-doped region andthe third p-doped region can reduce a first dark current flowing betweenthe first p-doped region and the first n-doped region, and the fourthn-doped region and the fourth p-doped region can reduce a second darkcurrent flowing between the second p-doped region and the second n-dopedregion relative to a comparable optical apparatus without the third andfourth n-doped regions and the third and fourth p-doped regions.

In some embodiments, the optical apparatus can further include a firstlayer supported by the first light absorption region, the first layerbeing different from the first light absorption region and covering thefirst trench and the second trench. The one or more first switches canfurther include a fifth n-doped region in contact with a second portionof the first p-doped region; and the one or more second switches canfurther include a sixth n-doped region in contact with a second portionthe second p-doped region.

In some embodiments, the first trench and the second trench can be atleast partially filled with a dielectric material.

Advantageous implementations may include one or more of the followingfeatures. Germanium is an efficient absorption material fornear-infrared wavelengths, which reduces the problem of slowphoto-carriers generated at a greater substrate depth when aninefficient absorption material, e.g., silicon, is used. For aphotodetector having p- and n-doped regions fabricated at two differentdepths, the photo-carrier transit distance is limited by the depth, andnot the width, of the absorption material. Consequently, if an efficientabsorption material with a short absorption length is used, the distancebetween the p- and n-doped regions can also be made short so that even asmall bias may create a strong field resulting into an increasedoperation speed. For such a photodetector, two groups of switches may beinserted and arranged laterally in an interdigitated arrangement, whichmay collect the photo-carriers at different optical phases for atime-of-flight system. An increased operation speed allows the use of ahigher modulation frequency in a time-of-flight system, giving a greaterdepth resolution. In a time-of-flight system where the peak intensity ofoptical pulses is increased while the duty cycle of the optical pulsesis decreased, the signal-to-noise ratio (and hence depth accuracy) canbe improved while maintaining the same power consumption for thetime-of-flight system. This is made possible when the operation speed isincreased so that the duty cycle of the optical pulses can be decreasedwithout distorting the pulse shape. In addition, by using germanium asthe absorption material, optical pulses at a wavelength longer than 1 μmcan be used. As longer NIR wavelengths (e.g. 1.31 μm, 1.4 μm, 1.55 μm)are generally accepted to be safer to the human eye, optical pulses canbe output at a higher intensity at longer wavelengths to improvesignal-to-noise-ratio (and hence a better depth accuracy) whilesatisfying eye-safety requirements.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other potentialfeatures and advantages will become apparent from the description, thedrawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C, and 1D are examples of a switched photodetector.

FIGS. 2A, 2B, 2C and 2D are examples of a switched photodetector.

FIGS. 3A, 3B, 3C, and 3D are examples of a switched photodetector.

FIGS. 4A, 4B, 4C, 4D, and 4E are examples of a switched photodetector.

FIGS. 5A-5C are examples of a photodetector.

FIGS. 5D-5K are examples of a switched photodetector.

FIGS. 6A-6B are examples of a switched photodetector.

FIGS. 7A-7B are cross-sectional views of example configurations ofmicrolenses integrated with photodetectors.

FIGS. 8A-8C are examples of a switch for a switched photodetector.

FIGS. 9A-9E are examples of an electrical terminal for a switchedphotodetector.

FIGS. 10A-10I are example configurations of a photodetector with anabsorption region and a substrate.

FIGS. 11A-11F are top and side views of examples of switchedphotodetectors.

FIGS. 12A-12H are top and side views of examples of switchedphotodetectors.

FIGS. 13A-13G are top and side views of examples of switchedphotodetectors.

FIGS. 14A-14B are top views of examples of switched photodetectors.

FIGS. 15A-15G are cross-sectional views of example configurations ofsensor pixel isolation.

FIGS. 16A-16J are cross-sectional views of example configurations ofphotodetectors.

FIGS. 17A-17E are cross-sectional views of example configurations ofabsorption region surface modification.

FIGS. 18A-18G show top and side views of example switchedphotodetectors.

FIGS. 19A-19H show top and side views of example switchedphotodetectors.

FIGS. 20A-20L show top and side views of example switchedphotodetectors.

FIGS. 21A-21F show top and side views of example switchedphotodetectors.

FIGS. 22A-22D show top and side views of example switchedphotodetectors.

FIGS. 23A-23B show top and side views of an example switchedphotodetector.

FIGS. 24A-24G show top and side views of example switchedphotodetectors.

FIGS. 25A-25H show top and side views of example switchedphotodetectors.

FIG. 26 is an example unit cell of rectangular photodetectors.

FIG. 27 is an example rectangular switched photodetector withphoto-transistor gain.

FIG. 28A is a block diagram of an example of an imaging system.

FIGS. 28B-28C show examples of techniques for determiningcharacteristics of an object using an imaging system.

FIG. 29 shows an example of a flow diagram for determiningcharacteristics of an object using an imaging system.

Like reference numbers and designations in the various drawings indicatelike elements. It is also to be understood that the various exemplaryembodiments shown in the figures are merely illustrative representationsand are not necessarily drawn to scale.

DETAILED DESCRIPTION

Photodetectors may be used to detect optical signals and convert theoptical signals to electrical signals that may be further processed byanother circuitry. In time-of-flight (TOF) applications, depthinformation of a three-dimensional object may be determined using aphase difference between a transmitted light pulse and a detected lightpulse. For example, a two-dimensional array of pixels may be used toreconstruct a three-dimensional image of a three-dimensional object,where each pixel may include one or more photodetectors for derivingphase information of the three-dimensional object. In someimplementations, time-of-flight applications use light sources havingwavelengths in the near-infrared (NIR) range. For example, alight-emitting-diode (LED) may have a wavelength of 850 nm, 940 nm, 1050nm, or 1.3 μm to 1.6 μm. Some photodetectors may use silicon as anabsorption material, but silicon is an inefficient absorption materialfor NIR wavelengths. Specifically, photo-carriers may be generateddeeply (e.g., greater than 10 μm in depth) in the silicon substrate, andthose photo-carriers may drift and/or diffuse to the photodetectorjunction slowly, which results in a decrease in the operation speed.Moreover, a small voltage swing is typically used to controlphotodetector operations in order to minimize power consumption. For alarge absorption area (e.g., 10 μm in diameter), the small voltage swingcan only create a small lateral/vertical field across the largeabsorption area, which affects the drift velocity of the photo-carriersbeing swept across the absorption area. The operation speed is thereforefurther limited. For TOF applications using NIR wavelengths, a switchedphotodetector with innovative design structures and/or with the use ofgermanium-silicon (GeSi) as an absorption material addresses thetechnical issues discussed above. In this application, the term“photodetector” may be used interchangeably with the term “opticalsensor”. In this application, the term “germanium-silicon (GeSi)” refersto a GeSi alloy with alloy composition ranging from 1% germanium (Ge),i.e., 99% silicon (Si), to 99% Ge, i.e., 1% of Si. In this application,the GeSi material may be grown using a blanket epitaxy, a selectiveepitaxy, or other applicable techniques. Furthermore, an absorptionlayer comprising the GeSi material may be formed on a planar surface, amesa top surface, or a trench bottom surface at least partiallysurrounded by an insulator (ex: oxide, nitrite), a semiconductor (ex:Si, Ge), or their combinations. Furthermore, a strained super latticestructure or a multiple quantum well structure including alternativelayers such as GeSi layers with two or more different alloy compositionsmay be used for the absorption layer. Furthermore, a Si layer or a GeSilayer with a low Ge concentration (e.g., <10%) may be used to passivatethe surface of a GeSi layer with a high Ge concentration (e.g., >50%),which may reduce a dark current or a leakage current at the surface ofthe GeSi layer with high Ge concentration.

FIG. 1A is an example switched photodetector 100 for converting anoptical signal to an electrical signal. The switched photodetector 100includes an absorption layer 106 fabricated on a substrate 102. Thesubstrate 102 may be any suitable substrate where semiconductor devicescan be fabricated on. For example, the substrate 102 may be a siliconsubstrate. The absorption layer 106 includes a first switch 108 and asecond switch 110.

In general, the absorption layer 106 receives an optical signal 112 andconverts the optical signal 112 into electrical signals. The absorptionlayer 106 may be intrinsic, p-type, or n-type. In some implementations,the absorption layer 106 may be formed from a p-type GeSi material. Theabsorption layer 106 is selected to have a high absorption coefficientat the desired wavelength range. For NIR wavelengths, the absorptionlayer 106 may be a GeSi mesa, where the GeSi absorbs photons in theoptical signal 112 and generates electron-hole pairs. The materialcomposition of germanium and silicon in the GeSi mesa may be selectedfor specific processes or applications. In some implementations, theabsorption layer 106 is designed to have a thickness t. For example, for850 nm or 940 nm wavelength, the thickness of the GeSi mesa may beapproximately 1 μm to have a substantial quantum efficiency. In someimplementations, the surface of the absorption layer 106 is designed tohave a specific shape. For example, the GeSi mesa may be circular,square, or rectangular depending on the spatial profile of the opticalsignal 112 on the surface of the GeSi mesa. In some implementations, theabsorption layer 106 is designed to have a lateral dimension d forreceiving the optical signal 112. For example, the GeSi mesa may have acircular or a rectangular shape, where d can range from 1 μm to 50 μm.

A first switch 108 and a second switch 110 have been fabricated in theabsorption layer 106. The first switch 108 is coupled to a first controlsignal 122 and a first readout circuit 124. The second switch 110 iscoupled to a second control signal 132 and a second readout circuit 134.In general, the first control signal 122 and the second control signal132 control whether the electrons or the holes generated by the absorbedphotons are collected by the first readout circuit 124 or the secondreadout circuit 134.

In some implementations, the first switch 108 and the second switch 110may be fabricated to collect electrons. In this case, the first switch108 includes a p-doped region 128 and an n-doped region 126. Forexample, the p-doped region 128 may have a p+ doping, where theactivated dopant concentration may be as high as a fabrication processmay achieve, e.g., the peak concentration may be about 5×10²⁰ cm⁻³ whenthe absorption layer 106 is germanium and doped with boron. In someimplementation, the doping concentration of the p-doped region 128 maybe lower than 5×10²⁰ cm⁻³ to ease the fabrication complexity at theexpense of an increased contact resistance. The n-doped region 126 mayhave an n+ doping, where the activated dopant concentration may be ashigh as a fabrication process may achieve, e.g., the peak concentrationmay be about 1×10²⁰ cm⁻³ when the absorption layer 106 is germanium anddoped with phosphorous. In some implementation, the doping concentrationof the n-doped region 126 may be lower than 1×10²⁰ cm⁻³ to ease thefabrication complexity at the expense of an increased contactresistance. The distance between the p-doped region 128 and the n-dopedregion 126 may be designed based on fabrication process design rules. Ingeneral, the closer the distance between the p-doped region 128 and then-doped region 126, the higher the switching efficiency of the generatedphoto-carriers. However, reducing of the distance between the p-dopedregion 128 and the n-doped region 126 may increase a dark currentassociated with a PN junction formed between the p-doped region 128 andthe n-doped region 126. As such, the distance may be set based on theperformance requirements of the switched photodetector 100. The secondswitch 110 includes a p-doped region 138 and an n-doped region 136. Thep-doped region 138 is similar to the p-doped region 128, and the n-dopedregion 136 is similar to the n-doped region 126.

In some implementations, the p-doped region 128 is coupled to the firstcontrol signal 122. For example, the p-doped region 128 may be coupledto a voltage source, where the first control signal 122 may be an ACvoltage signal from the voltage source. In some implementations, then-doped region 126 is coupled to the readout circuit 124. The readoutcircuit 124 may be in a three-transistor configuration consisting of areset gate, a source-follower, and a selection gate, a circuit includingfour or more transistors, or any suitable circuitry for processingcharges. In some implementations, the readout circuit 124 may befabricated on the substrate 102. In some other implementations, thereadout circuit 124 may be fabricated on another substrate andintegrated/co-packaged with the switched photodetector 100 via die/waferbonding or stacking.

The p-doped region 138 is coupled to the second control signal 132. Forexample, the p-doped region 138 may be coupled to a voltage source,where the second control signal 132 may be an AC voltage signal havingan opposite phase from the first control signal 122. In someimplementations, the n-doped region 136 is coupled to the readoutcircuit 134. The readout circuit 134 may be similar to the readoutcircuit 124.

The first control signal 122 and the second control signal 132 are usedto control the collection of electrons generated by the absorbedphotons. For example, when voltages are used, if the first controlsignal 122 is biased against the second control signal 132, an electricfield is created between the p-doped region 128 and the p-doped region138, and free electrons drift towards the p-doped region 128 or thep-doped region 138 depending on the direction of the electric field. Insome implementations, the first control signal 122 may be fixed at avoltage value V_(i), and the second control signal 132 may alternatebetween voltage values V_(i)±ΔV. The direction of the bias valuedetermines the drift direction of the electrons. Accordingly, when oneswitch (e.g., the first switch 108) is switched “on” (i.e., theelectrons drift towards the p-doped region 128), the other switch (e.g.,the second switch 110) is switched “off” (i.e. the electrons are blockedfrom the p-doped region 138). In some implementations, the first controlsignal 122 and the second control signal 132 may be voltages that aredifferential to each other.

In general, a difference (before equilibrium) between the Fermi level ofa p-doped region and the Fermi level of an n-doped region creates anelectric field between the two regions. In the first switch 108, anelectric field is created between the p-doped region 128 and the n-dopedregion 126. Similarly, in the second switch 110, an electric field iscreated between the p-doped region 138 and the n-doped region 136. Whenthe first switch 108 is switched “on” and the second switch 110 isswitched “off”, the electrons drift toward the p-doped region 128, andthe electric field between the p-doped region 128 and the n-doped region126 further carries the electrons to the n-doped region 126. The readoutcircuit 124 may then be enabled to process the charges collected by then-doped region 126. On the other hand, when the second switch 110 isswitched “on” and the first switch 108 is switched “off”, the electronsdrift toward the p-doped region 138, and the electric field between thep-doped region 138 and the n-doped region 136 further carries theelectrons to the n-doped region 136. The readout circuit 134 may then beenabled to process the charges collected by the n-doped region 136.

In some implementations, a voltage may be applied between the p-dopedand the n-doped regions of a switch to operate the switch in anavalanche regime to increase the sensitivity of the switchedphotodetector 100. For example, in the case of an absorption layer 106including GeSi, when the distance between the p-doped region 128 and then-doped region 126 is about 100 nm, it is possible to apply a voltagethat is less than 7 V to create an avalanche gain between the p-dopedregion 128 and the n-doped region 126.

In some implementations, the substrate 102 may be coupled to an externalcontrol 116. For example, the substrate 102 may be coupled to anelectrical ground, or a preset voltage less than the voltages at then-doped regions 126 and 136. In some other implementations, thesubstrate 102 may be floated and not coupled to any external control.

FIG. 1B is an example switched photodetector 160 for converting anoptical signal to an electrical signal. The switched photodetector 160is similar to the switched photodetector 100 in FIG. 1A, but that thefirst switch 108 and the second switch 110 further includes an n-wellregion 152 and an n-well region 154, respectively. In addition, theabsorption layer 106 may be a p-doped region. In some implementations,the doping level of the n-well regions 152 and 154 may range from 10¹⁵cm⁻³ to 10¹⁷ cm⁻³. The doping level of the absorption layer 106 mayrange from 10¹⁴ cm⁻³ to 10¹⁶ cm⁻³.

The arrangement of the p-doped region 128, the n-well region 152, thep-doped absorption layer 106, the n-well region 154, and the p-dopedregion 138 forms a PNPNP junction structure. In general, the PNPNPjunction structure reduces a leakage current from the first controlsignal 122 to the second control signal 132, or alternatively from thesecond control signal 132 to the first control signal 122. Thearrangement of the n-doped region 126, the p-doped absorption layer 106,and the n-doped region 136 forms an NPN junction structure. In general,the NPN junction structure reduces a charge coupling from the firstreadout circuit 124 to the second readout circuit 134, or alternativelyfrom the second readout circuit 134 to the first readout circuit 124.

In some implementations, the p-doped region 128 is formed entirelywithin the n-well region 152. In some other implementations, the p-dopedregion 128 is partially formed in the n-well region 152. For example, aportion of the p-doped region 128 may be formed by implanting thep-dopants in the n-well region 152, while another portion of the p-dopedregion 128 may be formed by implanting the p-dopants in the absorptionlayer 106. Similarly, in some implementations, the p-doped region 138 isformed entirely within the n-well region 154. In some otherimplementations, the p-doped region 138 is partially formed in then-well region 154. In some implementations, the depth of the n-wellregions 152 and 154 is shallower than the p-doped regions 128 and 138.

FIG. 1C is an example switched photodetector 170 for converting anoptical signal to an electrical signal. The switched photodetector 170is similar to the switched photodetector 100 in FIG. 1A, but that theabsorption layer 106 further includes an n-well region 156. In addition,the absorption layer 106 may be a p-doped region. In someimplementations, the doping level of the n-well region 156 may rangefrom 10¹⁵ cm⁻³ to 10¹⁷ cm⁻³. The doping level of the absorption layer106 may range from 10¹⁴ cm⁻³ to 10¹⁶ cm⁻³.

The arrangement of the p-doped region 128, the n-well region 156, andthe p-doped region 138 forms a PNP junction structure. In general, thePNP junction structure reduces a leakage current from the first controlsignal 122 to the second control signal 132, or alternatively from thesecond control signal 132 to the first control signal 122. Thearrangement of the n-doped region 126, the p-doped absorption layer 106,and the n-doped region 136 forms an NPN junction structure. In general,the NPN junction structure reduces a charge coupling from the firstreadout circuit 124 to the second readout circuit 134, or alternativelyfrom the second readout circuit 134 to the first readout circuit 124. Insome implementations, if the depth of the n-well region 156 is deep, thearrangement of the n-doped region 126, the p-doped absorption layer 106,the n-well region 156, the p-doped absorption layer 106, and the n-dopedregion 136 forms an NPNPN junction structure, which further reduces acharge coupling from the first readout circuit 124 to the second readoutcircuit 134, or alternatively from the second readout circuit 134 to thefirst readout circuit 124.

In some implementations, the p-doped regions 128 and 138 are formedentirely within the n-well region 156. In some other implementations,the p-doped regions 128 and 138 are partially formed in the n-wellregion 156. For example, a portion of the p-doped region 128 may beformed by implanting the p-dopants in the n-well region 156, whileanother portion of the p-doped region 128 may be formed by implantingthe p-dopants in the absorption layer 106. In some implementations, thedepth of the n-well region 156 is shallower than the p-doped regions 128and 138.

FIG. 1D is an example switched photodetector 180 for converting anoptical signal to an electrical signal. The switched photodetector 180is similar to the switched photodetector 100 in FIG. 1A, but that theswitched photodetector 150 further includes a p-well region 104 andn-well regions 142 and 144. In some implementations, the doping level ofthe n-well regions 142 and 144 may range from 10¹⁶ cm⁻³ to 10²⁰ cm⁻³.The doping level of the p-well region 104 may range from 10¹⁶ cm⁻³ to10²⁰ cm⁻³.

In some implementation, the absorption layer 106 may not completelyabsorb the incoming photons in the optical signal 112. For example, ifthe GeSi mesa does not completely absorb the incoming photons in the NIRoptical signal 112, the NIR optical signal 112 may penetrate into thesilicon substrate 102, where the silicon substrate 102 may absorb thepenetrated photons and generate photo-carriers deeply in the substratethat are slow to recombine. These slow photo-carriers negatively affectthe operation speed of the switched photodetector. Moreover, thephoto-carries generated in the silicon substrate 102 may be collected bythe neighboring pixels, which may cause unwanted signal cross-talksbetween the pixels. Furthermore, the photo-carriers generated in thesilicon substrate 102 may cause charging of the substrate 102, which maycause reliability issues in the switched photodiode.

To further remove the slow photo-carriers, the switched photodetector150 may include connections that short the n-well regions 142 and 144with the p-well region 104. For example, the connections may be formedby a silicide process or a deposited metal pad that connects the n-wellregions 142 and 144 with the p-well region 104. The shorting between then-well regions 142 and 144 and the p-well region 104 allows thephoto-carriers generated in the substrate 102 to be recombined at theshorted node, and therefore improves the operation speed and/orreliability of the switched photodetector. In some implementation, thep-well region 104 is used to passivate and/or minimize the electricfield around the interfacial defects between the absorptive layer 106and the substrate 102 in order to reduce the device dark current.

Although not shown in FIGS. 1A-1D, in some implementations, an opticalsignal may reach to the switched photodetector from the backside of thesubstrate 102. One or more optical components (e.g., microlens orlightguide) may be fabricated on the backside of the substrate 102 tofocus, collimate, defocus, filter, or otherwise manipulate the opticalsignal.

Although not shown in FIGS. 1A-1D, in some other implementations, thefirst switch 108 and the second switch 110 may alternatively befabricated to collect holes instead of electrons. In this case, thep-doped region 128 and the p-doped region 138 would be replaced byn-doped regions, and the n-doped region 126 and the n-doped region 136would be replaced by p-doped regions. The n-well regions 142, 144, 152,154, and 156 would be replaced by p-well regions. The p-well region 104would be replaced by an n-well region.

Although not shown in FIGS. 1A-1D, in some implementations, theabsorption layer 106 may be bonded to a substrate after the fabricationof the switched photodetector 100, 160, 170, and 180. The substrate maybe any material that allows the transmission of the optical signal 112to reach to the switched photodetector. For example, the substrate maybe polymer or glass. In some implementations, one or more opticalcomponents (e.g., microlens or lightguide) may be fabricated on thecarrier substrate to focus, collimate, defocus, filter, or otherwisemanipulate the optical signal 112.

Although not shown in FIGS. 1A-1D, in some implementations, the switchedphotodetector 100, 160, 170, and 180 may be bonded (ex: metal to metalbonding, oxide to oxide bonding, hybrid bonding) to a second substratewith circuits including control signals, and/or, readout circuits,and/or phase lock loop (PLL), and/or analog to digital converter (ADC).A metal layer may be deposited on top of the switched photodetector thatmay be used as a reflector to reflect the optical signal incident fromthe backside of the substrate 102. Adding such a mirror like metal layermay increase the absorption efficiency (quantum efficiency) of theabsorption layer 106. For example, the absorption efficiency ofphotodetectors operating at a longer NIR wavelength between 1.0 μm and1.6 μm may be significantly improved by addition of a reflecting metallayer. An oxide layer may be included between the metal layer and theabsorptive layer to increase the reflectivity. The metal layer may alsobe used as the bonding layer for the wafer-bonding process. In someimplementations, one or more switches similar to 108 and 110 can beadded for interfacing control signals/readout circuits.

Although not shown in FIG. 1A-1D, in some implementations, theabsorption layer 106 may be partially or fully embedded/recessed in thesubstrate 102 to relieve the surface topography and so ease thefabrication process. An example of the embedment technique is describedin U.S. Patent Publication No. US20170040362 A1 titled“Germanium-Silicon Light Sensing Apparatus,” which is fully incorporatedby reference herein.

FIG. 2A is an example switched photodetector 200 for converting anoptical signal to an electrical signal, where the first switch 208 andthe second switch 210 are fabricated on a substrate 202. The switchedphotodetector 200 includes an absorption layer 206 fabricated on asubstrate 202. The substrate 202 may be any suitable substrate wheresemiconductor devices can be fabricated on. For example, the substrate202 may be a silicon substrate.

In general, the absorption layer 206 receives an optical signal 212 andconverts the optical signal 212 into electrical signals. The absorptionlayer 206 is similar to the absorption layer 106. The absorption layer206 may be intrinsic, p-type, or n-type. In some implementations, theabsorption layer 206 may be formed from a p-type GeSi material. In someimplementations, the absorption layer 206 may include a p-doped region209. The p-doped region 209 may repel the photo-electrons from theabsorption layer 206 to the substrate 202 and thereby increase theoperation speed. For example, the p-doped region 209 may have a p+doping, where the dopant concentration is as high as a fabricationprocess may achieve, e.g., the peak concentration may be about 5×10²⁰cm⁻³ when the absorption layer 206 is germanium and doped with boron. Insome implementation, the doping concentration of the p-doped region 209may be lower than 5×10²⁰ cm⁻³ to ease the fabrication complexity at theexpense of an increased contact resistance. In some implementations, thep-doped region 209 may be a graded p-doped region.

A first switch 208 and a second switch 210 have been fabricated in thesubstrate 202. The first switch 208 is coupled to a first control signal222 and a first readout circuit 224. The second switch 210 is coupled toa second control signal 232 and a second readout circuit 234. Ingeneral, the first control signal 222 and the second control signal 232control whether the electrons or the holes generated by the absorbedphotons are collected by the first readout circuit 224 or the secondreadout circuit 234. The first control signal 222 is similar to thefirst control signal 122. The second control signal 232 is similar tothe second control signal 132. The first readout circuit 224 is similarto the first readout circuit 124. The second readout circuit 234 issimilar to the second readout circuit 134.

In some implementations, the first switch 208 and the second switch 210may be fabricated to collect electrons generated by the absorption layer206. In this case, the first switch 208 includes a p-doped region 228and an n-doped region 226. For example, the p-doped region 228 may havea p+ doping, where the activated dopant concentration may be as high asa fabrication process may achieve, e.g., the peak concentration may beabout 2×10²⁰ cm⁻³ when the substrate 202 is silicon and doped withboron. In some implementation, the doping concentration of the p-dopedregion 228 may be lower than 2×10²⁰ cm⁻³ to ease the fabricationcomplexity at the expense of an increased contact resistance. Then-doped region 226 may have an n+ doping, where the activated dopantconcentration may be as high as a fabrication process may achieve, e.g.,the peak concentration may be about 5×10²⁰ cm⁻³ when the substrate 202is silicon and doped with phosphorous. In some implementation, thedoping concentration of the n-doped region 226 may be lower than 5×10²⁰cm⁻³ to ease the fabrication complexity at the expense of an increasedcontact resistance. The distance between the p-doped region 228 and then-doped region 226 may be designed based on fabrication process designrules. In general, the closer the distance between the p-doped region228 and the n-doped region 226, the higher the switching efficiency ofthe generated photo-carriers. The second switch 210 includes a p-dopedregion 238 and an n-doped region 236. The p-doped region 238 is similarto the p-doped region 228, and the n-doped region 236 is similar to then-doped region 226.

In some implementations, the p-doped region 228 is coupled to the firstcontrol signal 222. The n-doped region 226 is coupled to the readoutcircuit 224. The p-doped region 238 is coupled to the second controlsignal 232. The n-doped region 236 is coupled to the readout circuit234. The first control signal 222 and the second control signal 232 areused to control the collection of electrons generated by the absorbedphotons. For example, when the absorption layer 206 absorbs photons inthe optical signal 212, electron-hole pairs are generated and drift ordiffuse into the substrate 202. When voltages are used, if the firstcontrol signal 222 is biased against the second control signal 232, anelectric field is created between the p-doped region 228 and the p-dopedregion 238, and free electrons from the absorption layer 206 drifttowards the p-doped region 228 or the p-doped region 238 depending onthe direction of the electric field. In some implementations, the firstcontrol signal 222 may be fixed at a voltage value V_(i), and the secondcontrol signal 232 may alternate between voltage values V_(i)±ΔV. Thedirection of the bias value determines the drift direction of theelectrons. Accordingly, when one switch (e.g., the first switch 208) isswitched “on” (i.e., the electrons drift towards the p-doped region228), the other switch (e.g., the second switch 210) is switched “off”(i.e., the electrons are blocked from the p-doped region 238). In someimplementations, the first control signal 222 and the second controlsignal 232 may be voltages that are differential to each other.

In the first switch 208, an electric field is created between thep-doped region 228 and the n-doped region 226. Similarly, in the secondswitch 210, an electric field is created between the p-doped region 238and the n-doped region 236. When the first switch 208 is switched “on”and the second switch 210 is switched “off”, the electrons drift towardthe p-doped region 228, and the electric field between the p-dopedregion 228 and the n-doped region 226 further carries the electrons tothe n-doped region 226. The readout circuit 224 may then be enabled toprocess the charges collected by the n-doped region 226. On the otherhand, when the second switch 210 is switched “on” and the first switch208 is switched “off”, the electrons drift toward the p-doped region238, and the electric field between the p-doped region 238 and then-doped region 236 further carries the electrons to the n-doped region236. The readout circuit 234 may then be enabled to process the chargescollected by the n-doped region 236.

In some implementations, a voltage may be applied between the p-dopedand the n-doped regions of a switch to operate the switch in anavalanche regime to increase the sensitivity of the switchedphotodetector 200. For example, in the case of a substrate 202 includingGeSi, when the distance between the p-doped region 228 and the n-dopedregion 226 is about 100 nm, it is possible to apply a voltage that isless than 7 V to create an avalanche gain between the p-doped region 228and the n-doped region 226.

In some implementations, the p-doped region 209 may be coupled to anexternal control 214. For example, the p-doped region 209 may be coupledto ground. In some implementations, the p-doped region 209 may befloated and not coupled to any external control. In someimplementations, the substrate 202 may be coupled to an external control216. For example, the substrate 202 may be coupled to an electricalground, or a preset voltage less than the voltages at the n-dopedregions 226 and 236. In some other implementations, the substrate 202may be floated and not coupled to any external control.

FIG. 2B is an example switched photodetector 250 for converting anoptical signal to an electrical signal. The switched photodetector 250is similar to the switched photodetector 200 in FIG. 2A, but that thefirst switch 208 and the second switch 210 further includes an n-wellregion 252 and an n-well region 254, respectively. In addition, theabsorption layer 206 may be a p-doped region and the substrate 202 maybe a p-doped substrate. In some implementations, the doping level of then-well regions 252 and 254 may range from 10¹⁵ cm⁻³ to 10¹⁷ cm⁻³. Thedoping level of the absorption layer 206 and the substrate 202 may rangefrom 10¹⁴ cm⁻³ to 10¹⁶ cm⁻³.

The arrangement of the p-doped region 228, the n-well region 252, thep-doped substrate 202, the n-well region 254, and the p-doped region 238forms a PNPNP junction structure. In general, the PNPNP junctionstructure reduces a leakage current from the first control signal 222 tothe second control signal 232, or alternatively from the second controlsignal 232 to the first control signal 222. The arrangement of then-doped region 226, the p-doped substrate 202, and the n-doped region236 forms an NPN junction structure. In general, the NPN junctionstructure reduces a charge coupling from the first readout circuit 224to the second readout circuit 234, or alternatively from the secondreadout circuit 234 to the first readout circuit 224.

In some implementations, the p-doped region 228 is formed entirelywithin the n-well region 252. In some other implementations, the p-dopedregion 228 is partially formed in the n-well region 252. For example, aportion of the p-doped region 228 may be formed by implanting thep-dopants in the n-well region 252, while another portion of the p-dopedregion 228 may be formed by implanting the p-dopants in the substrate202. Similarly, in some implementations, the p-doped region 238 isformed entirely within the n-well region 254. In some otherimplementations, the p-doped region 238 is partially formed in then-well region 254. In some implementations, the depth of the n-wellregions 252 and 254 is shallower than the p-doped regions 228 and 238.

FIG. 2C is an example switched photodetector 260 for converting anoptical signal to an electrical signal. The switched photodetector 260is similar to the switched photodetector 200 in FIG. 2A, but that thesubstrate 202 further includes an n-well region 244. In addition, theabsorption layer 206 may be a p-doped region and the substrate 202 maybe a p-doped substrate. In some implementations, the doping level of then-well region 244 may range from 10¹⁵ cm⁻³ to 10¹⁷ cm⁻³. The dopinglevel of the absorption layer 206 and the substrate 202 may range from10¹⁴ cm⁻³ to 10¹⁶ cm⁻³.

The arrangement of the p-doped region 228, the n-well region 244, andthe p-doped region 238 forms a PNP junction structure. In general, thePNP junction structure reduces a leakage current from the first controlsignal 222 to the second control signal 232, or alternatively from thesecond control signal 232 to the first control signal 222. Thearrangement of the n-doped region 226, the p-doped substrate 202, andthe n-doped region 236 forms an NPN junction structure. In general, theNPN junction structure reduces a charge coupling from the first readoutcircuit 224 to the second readout circuit 234, or alternatively from thesecond readout circuit 234 to the first readout circuit 224. In someimplementations, if the depth of the n-well 244 is deep, the arrangementof the n-doped region 226, the p-doped substrate 202, the n-well region244, the p-doped substrate 202, and the n-doped region 236 forms anNPNPN junction structure, which further reduces a charge coupling fromthe first readout circuit 224 to the second readout circuit 234, oralternatively from the second readout circuit 234 to the first readoutcircuit 224. In some implementations, the n-well region 244 alsoeffectively reduces the potential energy barrier perceived by theelectrons flowing from the absorption layer 206 to the substrate 202.

In some implementations, the p-doped regions 228 and 238 are formedentirely within the n-well region 244. In some other implementations,the p-doped regions 228 and 238 are partially formed in the n-wellregion 244. For example, a portion of the p-doped region 228 may beformed by implanting the p-dopants in the n-well region 244, whileanother portion of the p-doped region 228 may be formed by implantingthe p-dopants in the substrate 202. In some implementations, the depthof the n-well region 244 is shallower than the p-doped regions 228 and238.

FIG. 2D is an example switched photodetector 270 for converting anoptical signal to an electrical signal. The switched photodetector 270is similar to the switched photodetector 200 in FIG. 2A, but that theswitched photodetector 270 further includes one or more p-well regions246 and one or more p-well regions 248. In some implementations, the oneor more p-well regions 246 and the one or more p-well regions 248 may bepart of a ring structure that surrounds the first switch 208 and thesecond switch 210. In some implementations, the doping level of the oneor more p-well regions 246 and 248 may range from 10¹⁵ cm⁻³ to 10²⁰cm⁻³. The one or more p-well regions 246 and 248 may be used as anisolation of photo-electrons from the neighboring pixels.

Although not shown in FIG. 2A-2D, in some implementations, an opticalsignal may reach to the switched photodetector from the backside of thesubstrate 202. One or more optical components (e.g., microlens orlightguide) may be fabricated on the backside of the substrate 202 tofocus, collimate, defocus, filter, or otherwise manipulate the opticalsignal.

Although not shown in FIG. 2A-2D, in some other implementations, thefirst switch 208 and the second switch 210 may alternatively befabricated to collect holes instead of electrons. In this case, thep-doped region 228, the p-doped region 238, and the p-doped region 209would be replaced by n-doped regions, and the n-doped region 226 and then-doped region 236 would be replaced by p-doped regions. The n-wellregions 252, 254, and 244 would be replaced by p-well regions. Thep-well regions 246 and 248 would be replaced by n-well regions.

Although not shown in FIG. 2A-2D, in some implementations, theabsorption layer 206 may be bonded to a substrate after the fabricationof the switched photodetector 200, 250, 260, and 270. The carriersubstrate may be any material that allows the transmission of theoptical signal 212 to reach to the switched photodetector. For example,the substrate may be polymer or glass. In some implementations, one ormore optical components (e.g., microlens or lightguide) may befabricated on the carrier substrate to focus, collimate, defocus,filter, or otherwise manipulate the optical signal 212.

Although not shown in FIGS. 2A-2D, in some implementations, the switchedphotodetector 200, 250, 260, and 270 may be bonded (e.g., metal to metalbonding, oxide to oxide bonding, hybrid bonding) to a second substratewith circuits including control signals, and/or, readout circuits,and/or phase lock loop (PLL), and/or analog to digital converter (ADC).A metal layer may be deposited on top of the switched photodetector thatmay be used as a reflector to reflect the optical signal incident fromthe backside of the substrate 202. Adding such a mirror like metal layermay increase the absorption efficiency (quantum efficiency) of theabsorption layer 206. For example, the absorption efficiency ofphotodetectors operating at a longer NIR wavelength between 1.0 μm and1.6 μm may be significantly improved by addition of a reflecting metallayer. An oxide layer may be included between the metal layer and theabsorptive layer to increase the reflectivity. The metal layer may alsobe used as the bonding layer for the wafer-bonding process. In someimplementations, one or more switches similar to 208 and 210 can beadded for interfacing control signals/readout circuits.

Although not shown in FIG. 2A-2D, in some implementations, theabsorption layer 206 may be partially or fully embedded/recessed in thesubstrate 202 to relieve the surface topography and so ease thefabrication process. An example of the embedment technique is describedin U.S. Patent Publication No. US20170040362 A1.

FIG. 3A is an example switched photodetector 300 for converting anoptical signal to an electrical signal, where first switches 308 a and308 b, and second switch 310 a and 310 b are fabricated in a verticalarrangement on a substrate 302. One characteristic with the switchedphotodetector 100 or the switched photodetector 200 is that the largerthe optical window size d, the longer the photo-electron transit timerequired for an electron drifts or diffuses from one switch to the otherswitch. The operation speed of the photodetector may therefore beaffected. The switched photodetector 300 may further improve theoperation speed by arranging the p-doped regions and the n-doped regionsof the switches in a vertical arrangement. Using this verticalarrangement, the photo-electron transit distance is limited mostly bythe thickness t (e.g., ˜1 μm) of the absorption layer instead of thewindow size d (e.g., ˜10 μm) of the absorption layer. The switchedphotodetector 300 includes an absorption layer 306 fabricated on asubstrate 302. The substrate 302 may be any suitable substrate wheresemiconductor devices can be fabricated on. For example, the substrate302 may be a silicon substrate.

In general, the absorption layer 306 receives an optical signal 312 andconverts the optical signal 312 into electrical signals. The absorptionlayer 306 is similar to the absorption layer 206. The absorption layer306 may be intrinsic, p-type, or n-type. In some implementations, theabsorption layer 306 may be formed from a p-type GeSi material. In someimplementations, the absorption layer 306 may include a p-doped region309. The p-doped region 309 is similar to the p-doped region 209.

First switches 308 a and 308 b, and second switches 310 a and 310 b havebeen fabricated in the substrate 302. Notably, although FIG. 3A onlyshows two first switches 308 a and 308 b and two second switches 310 aand 310 b, the number of first switches and second switches may be moreor less. The first switches 308 a and 308 b are coupled to a firstcontrol signal 322 and a first readout circuit 324. The second switches310 a and 310 b are coupled to a second control signal 332 and a secondreadout circuit 334.

In general, the first control signal 322 and the second control signal332 control whether the electrons or the holes generated by the absorbedphotons are collected by the first readout circuit 324 or the secondreadout circuit 334. The first control signal 322 is similar to thefirst control signal 122. The second control signal 332 is similar tothe second control signal 132. The first readout circuit 324 is similarto the first readout circuit 124. The second readout circuit 334 issimilar to the second readout circuit 134. In some implementations, thefirst switches 308 a and 308 b, and the second switches 310 a and 310 bmay be fabricated to collect electrons generated by the absorption layer306. In this case, the first switches 308 a and 308 b include p-dopedregions 328 a and 328 b, and n-doped regions 326 a and 326 b,respectively. For example, the p-doped regions 328 a and 328 b may havea p+ doping, where the activated dopant concentration may be as high asa fabrication process may achieve, e.g., the peak concentration may beabout 2×10²⁰ cm⁻³ when the substrate 302 is silicon and doped withboron. In some implementation, the doping concentration of the p-dopedregions 328 a and 328 b may be lower than 2×10²⁰ cm⁻³ to ease thefabrication complexity at the expense of an increased contactresistance. The n-doped regions 326 a and 326 b may have an n+ doping,where the activated dopant concentration may be as high as a fabricationprocess may achieve, e.g., the peak concentration may be about 5×10²⁰cm⁻³ when the substrate 302 is silicon and doped with phosphorous. Insome implementation, the doping concentration of the n-doped regions 326a and 326 b may be lower than 5×10²⁰ cm⁻³ to ease the fabricationcomplexity at the expense of an increased contact resistance. Thedistance between the p-doped region 328 a and the n-doped region 326 amay be designed based on fabrication process design rules. For example,the distance between the p-doped region 328 a and the n-doped region 326a may be controlled by the energies associated with the dopant implants.In general, the closer the distance between the p-doped regions 328a/328 b and the n-doped regions 326 a/326 b, the higher the switchingefficiency of the generated photo-carriers. The second switches 310 aand 310 b includes p-doped regions 338 a and 338 b, and n-doped regions336 a and 336 b, respectively. The p-doped regions 338 a/338 b aresimilar to the p-doped regions 328 a/328 b, and the n-doped regions 336a/336 b are similar to the n-doped region 326 a/326 b.

In some implementations, the p-doped regions 328 a and 328 b are coupledto the first control signal 322. The n-doped regions 326 a and 326 b arecoupled to the readout circuit 324. The p-doped regions 338 a and 338 bare coupled to the second control signal 332. The n-doped regions 336 aand 336 b are coupled to the readout circuit 334. The first controlsignal 322 and the second control signal 332 are used to control thecollection of electrons generated by the absorbed photons. For example,when the absorption layer 306 absorbs photons in the optical signal 312,electron-hole pairs are generated and drift or diffuse into thesubstrate 302. When voltages are used, if the first control signal 322is biased against the second control signal 332, electric fields arecreated between the p-doped region 309 and the p-doped regions 328 a/328b or the p-doped regions 338 a/338 b, and free electrons from theabsorption layer 306 drift towards the p-doped regions 328 a/328 b orthe p-doped regions 338 a/338 b depending on the directions of theelectric fields. In some implementations, the first control signal 322may be fixed at a voltage value V_(i), and the second control signal 332may alternate between voltage values V_(i)±ΔV. The direction of the biasvalue determines the drift direction of the electrons. Accordingly, whenone group of switches (e.g., first switches 308 a and 308 b) areswitched “on” (i.e., the electrons drift towards the p-doped regions 328a and 328 b), the other group of switches (e.g., the second switches 310a and 310 b) are switched “off” (i.e., the electrons are blocked fromthe p-doped regions 338 a and 338 b). In some implementations, the firstcontrol signal 322 and the second control signal 332 may be voltagesthat are differential to each other.

In each of the first switches 308 a/308 b, an electric field is createdbetween the p-doped region 328 a/328 b and the n-doped region 326 a/326b. Similarly, in each of the second switches 310 a/310 b, an electricfield is created between the p-doped region 338 a/338 b and the n-dopedregion 336 a/336 b. When the first switches 308 a and 308 b are switched“on” and the second switches 310 a and 310 b are switched “off”, theelectrons drift toward the p-doped regions 328 a and 328 b, and theelectric field between the p-doped region 328 a and the n-doped region326 a further carries the electrons to the n-doped region 326 a.Similarly, the electric field between the p-doped region 328 b and then-doped region 326 b further carries the electrons to the n-doped region326 b. The readout circuit 324 may then be enabled to process thecharges collected by the n-doped regions 326 a and 326 b. On the otherhand, when the second switches 310 a and 310 b are switched “on” and thefirst switches 308 a and 308 b are switched “off”, the electrons drifttoward the p-doped regions 338 a and 338 b, and the electric fieldbetween the p-doped region 338 a and the n-doped region 336 a furthercarries the electrons to the n-doped region 336 a. Similarly, theelectric field between the p-doped region 338 b and the n-doped region336 b further carries the electrons to the n-doped region 336 b. Thereadout circuit 334 may then be enabled to process the amount of chargescollected by the n-doped regions 336 a and 336 b.

In some implementations, a voltage may be applied between the p-dopedand the n-doped regions of a switch to operate the switch in anavalanche regime to increase the sensitivity of the switchedphotodetector 300. For example, in the case of a substrate 302 includingGeSi, when the distance between the p-doped region 328 a and the n-dopedregion 326 a is about 100 nm, it is possible to apply a voltage that isless than 7 V to create an avalanche gain between the p-doped region 328a and the n-doped region 326 a.

In some implementations, the p-doped region 309 may be coupled to anexternal control 314. For example, the p-doped region 309 may be coupledto ground. In some implementations, the p-doped region 309 may befloated and not coupled to any external control. In someimplementations, the substrate 302 may be coupled to an external control316. For example, the substrate 302 may be coupled to an electricground, or a preset voltage less than the voltages at the n-dopedregions 326 and 336. In some other implementations, the substrate 302may be floated and not coupled to any external control.

FIG. 3B is an example switched photodetector 360 for converting anoptical signal to an electrical signal. The switched photodetector 360is similar to the switched photodetector 300 in FIG. 3A, but that theswitched photodetector 360 further includes an n-well region 344. Inaddition, the absorption layer 360 may be a p-doped region and thesubstrate 302 may be a p-doped substrate. In some implementations, thedoping level of the n-well region 344 may range from 10¹⁵ cm⁻³ to 10¹⁷cm⁻³. The doping level of the absorption layer 360 and the substrate 302may range from 10¹⁴ cm⁻³ to 10¹⁶ cm⁻³.

The arrangement of the p-doped region 328 a, the n-well region 344, andthe p-doped region 338 a forms a PNP junction structure. Similarly, thearrangement of the p-doped region 328 b, the n-well region 344, and thep-doped region 338 b forms another PNP junction structure. In general,the PNP junction structure reduces a leakage current from the firstcontrol signal 322 to the second control signal 332, or alternativelyfrom the second control signal 332 to the first control signal 322. Thearrangement of the n-doped region 326 a, the p-doped substrate 302, andthe n-doped region 336 a forms an NPN junction structure. Similarly, thearrangement of the n-doped region 326 b, the p-doped substrate 302, andthe n-doped region 336 b forms an NPN junction structure. In general,the NPN junction structure reduces a charge coupling from the firstreadout circuit 324 to the second readout circuit 334, or alternativelyfrom the second readout circuit 334 to the first readout circuit 324. Insome implementations, the n-well region 344 also effectively reduces thepotential energy barrier perceived by the electrons flowing from theabsorption layer 306 to the substrate 302.

In some implementations, the p-doped regions 328 a, 338 a, 328 b, and338 b are formed entirely within the n-well region 344. In some otherimplementations, the p-doped regions 328 a, 338 a, 328 b, and 338 b arepartially formed in the n-well region 344. For example, a portion of thep-doped region 328 a may be formed by implanting the p-dopants in then-well region 344, while another portion of the p-doped region 328 a maybe formed by implanting the p-dopants in the substrate 302. In someimplementations, the depth of the n-well region 344 is shallower thanthe p-doped regions 328 a, 338 a, 328 b, and 338 b.

FIG. 3C is an example switched photodetector 370 for converting anoptical signal to an electrical signal. The switched photodetector 370is similar to the switched photodetector 300 in FIG. 3A, but that theswitched photodetector 370 further includes one or more p-well regions346 and one or more p-well regions 348. In some implementations, the oneor more p well regions 346 and the one or more p-well regions 348 may bepart of a ring structure that surrounds the first switches 308 a and 308b, and the second switches 310 a and 310 b. In some implementations, thedoping level of the one or more p-well regions may range from 10¹⁵ cm⁻³to 10²⁰ cm⁻³. The one or more p-well regions 346 and 348 may be used asan isolation of photo-electrons from the neighboring pixels.

FIG. 3D shows cross-sectional views of the example switchedphotodetector 380. FIG. 3D shows that the p-doped regions 328 a and 328b of the first switches 308 a and 308 b, and the p-doped regions 338 aand 338 b of the second switches 310 a and 310 b may be arranged on afirst plane 362 of the substrate 302 in an interdigitated arrangement.FIG. 3D further shows that the n-doped regions 326 a and 326 b of thefirst switches 308 a and 308 b, and the n-doped regions 336 a and 336 bof the second switches 310 a and 310 b may be arranged on a second plane364 of the substrate 302 in an interdigitated arrangement.

Although not shown in FIG. 3A-3D, in some implementations, an opticalsignal may reach to the switched photodetector from the backside of thesubstrate 302. One or more optical components (e.g., microlens orlightguide) may be fabricated on the backside of the substrate 302 tofocus, collimate, defocus, filter, or otherwise manipulate the opticalsignal.

Although not shown in FIG. 3A-3D, in some other implementations, thefirst switches 308 a and 308 b, and the second switches 310 a and 310 bmay alternatively be fabricated to collect holes instead of electrons.In this case, the p-doped regions 328 a and 328 b, the p-doped regions338 a and 338 b, and the p-doped region 309 would be replaced by n-dopedregions, and the n-doped regions 326 a and 326 b, and the n-dopedregions 336 a and 336 b would be replaced by p-doped regions. The n-wellregion 344 would be replaced by a p-well region. The p-well regions 346and 348 would be replaced by n-well regions.

Although not shown in FIG. 3A-3D, in some implementations, theabsorption layer 306 may be bonded to a substrate after the fabricationof the switched photodetector 300, 360, 370, and 380. The substrate maybe any material that allows the transmission of the optical signal 312to reach to the switched photodetector. For example, the substrate maybe polymer or glass. In some implementations, one or more opticalcomponents (e.g., microlens or lightguide) may be fabricated on thecarrier substrate to focus, collimate, defocus, filter, or otherwisemanipulate the optical signal 312.

Although not shown in FIGS. 3A-3D, in some implementations, the switchedphotodetector 300, 360, 370, and 380 may be bonded (ex: metal to metalbonding, oxide to oxide bonding, hybrid bonding) to a second substratewith circuits including control signals, and/or, readout circuits,and/or phase lock loop (PLL), and/or analog to digital converter (ADC).A metal layer may be deposited on top of the switched photodetector thatmay be used as a reflector to reflect the optical signal incident fromthe backside of the substrate 302. Adding such a mirror like metal layermay increase the absorption efficiency (quantum efficiency) of theabsorption layer 306. For example, the absorption efficiency ofphotodetectors operating at a longer NIR wavelength between 1.0 μm and1.6 μm may be significantly improved by addition of a reflecting metallayer. An oxide layer may be included between the metal layer and theabsorptive layer to increase the reflectivity. The metal layer may alsobe used as the bonding layer for the wafer-bonding process. In someimplementations, one or more switches similar to 308 a (or 308 b) and310 a (or 310 b) can be added for interfacing control signals/readoutcircuits.

Although not shown in FIG. 3A-3D, in some implementations, theabsorption layer 306 may be partially or fully embedded/recessed in thesubstrate 302 to relieve the surface topography and so ease thefabrication process. An example of the embedment technique is describedin U.S. Patent Publication No. US20170040362 A1.

FIG. 4A is an example switched photodetector 400 for converting anoptical signal to an electrical signal. The switched photodetector 400includes an absorption layer 406 fabricated on a substrate 402. Thesubstrate 402 may be any suitable substrate where semiconductor devicescan be fabricated on. For example, the substrate 402 may be a siliconsubstrate. The absorption layer 406 includes a first switch 408 and asecond switch 410.

In general, the absorption layer 406 receives an optical signal 412 andconverts the optical signal 412 into electrical signals. The absorptionlayer 406 may be intrinsic, p-type, or n-type. In some implementations,the absorption layer 406 may be formed from a p-type GeSi material. Theabsorption layer 406 is selected to have a high absorption coefficientat the desired wavelength range. For NIR wavelengths, the absorptionlayer 406 may be a GeSi mesa, where the GeSi absorbs photons in theoptical signal 412 and generates electron-hole pairs. The materialcomposition of germanium and silicon in the GeSi mesa may be selectedfor specific processes or applications. In some implementations, theabsorption layer 406 is designed to have a thickness t. For example, for850 nm or 940 nm wavelength, the thickness of the GeSi mesa may beapproximately 1 μm to have a substantial quantum efficiency. In someimplementations, the surface of the absorption layer 406 is designed tohave a specific shape. For example, the GeSi mesa may be circular,square, or rectangular depending on the spatial profile of the opticalsignal 412 on the surface of the GeSi mesa. In some implementations, theabsorption layer 406 is designed to have a lateral dimension d forreceiving the optical signal 412. For example, the GeSi mesa may have acircular or a rectangular shape, where d can range from 1 μm to 50 μm.

A first switch 408 and a second switch 410 have been fabricated in theabsorption layer 406 and the substrate 402. The first switch 408 iscoupled to a first control signal 422 and a first readout circuit 424.The second switch 410 is coupled to a second control signal 432 and asecond readout circuit 434. In general, the first control signal 422 andthe second control signal 432 control whether the electrons or the holesgenerated by the absorbed photons are collected by the first readoutcircuit 424 or the second readout circuit 434.

In some implementations, the first switch 408 and the second switch 410may be fabricated to collect electrons. In this case, the first switch408 includes a p-doped region 428 implanted in the absorption layer 406and an n-doped region 426 implanted in the substrate 402. For example,the p-doped region 428 may have a p+ doping, where the activated dopantconcentration may be as high as a fabrication process may achieve, e.g.,the peak concentration may be about 5×10²⁰ cm⁻³ when the absorptionlayer 106 is germanium and doped with boron. In some implementation, thedoping concentration of the p-doped region 428 may be lower than 5×10²⁰cm⁻³ to ease the fabrication complexity at the expense of an increasedcontact resistance. The n-doped region 426 may have an n+ doping, wherethe activated dopant concentration may be as high as a fabricationprocess may achieve, e.g., e.g., the peak concentration may be about5×10²⁰ cm⁻³ when the substrate 402 is silicon and doped withphosphorous. In some implementation, the doping concentration of then-doped region 426 may be lower than 5×10²⁰ cm⁻³ to ease the fabricationcomplexity at the expense of an increased contact resistance. Thedistance between the p-doped region 428 and the n-doped region 426 maybe designed based on fabrication process design rules. In general, thecloser the distance between the p-doped region 428 and the n-dopedregion 426, the higher the switching efficiency of the generatedphoto-carriers. The second switch 410 includes a p-doped region 438 andan n-doped region 436. The p-doped region 438 is similar to the p-dopedregion 428, and the n-doped region 436 is similar to the n-doped region426.

In some implementations, the p-doped region 428 is coupled to the firstcontrol signal 422. For example, the p-doped region 428 may be coupledto a voltage source, where the first control signal 422 may be an ACvoltage signal from the voltage source. In some implementations, then-doped region 426 is coupled to the readout circuit 424. The readoutcircuit 424 may be in a three-transistor configuration consisting of areset gate, a source-follower, and a selection gate, a circuit includingfour or more transistors, or any suitable circuitry for processingcharges. In some implementations, the readout circuit 424 may befabricated on the substrate 402. In some other implementations, thereadout circuit 424 may be fabricated on another substrate andintegrated/co-packaged with the switched photodetector 400 via die/waferbonding or stacking.

The p-doped region 438 is coupled to the second control signal 432. Forexample, the p-doped region 438 may be coupled to a voltage source,where the second control signal 432 may be an AC voltage signal havingan opposite phase from the first control signal 422. In someimplementations, the n-doped region 436 is coupled to the readoutcircuit 434. The readout circuit 434 may be similar to the readoutcircuit 424.

The first control signal 422 and the second control signal 432 are usedto control the collection of electrons generated by the absorbedphotons. For example, when voltages are used, if the first controlsignal 422 is biased against the second control signal 432, an electricfield is created between the p-doped region 428 and the p-doped region438, and free electrons drift towards the p-doped region 428 or thep-doped region 438 depending on the direction of the electric field. Insome implementations, the first control signal 422 may be fixed at avoltage value V_(i), and the second control signal 432 may alternatebetween voltage values V_(i)±ΔV. The direction of the bias valuedetermines the drift direction of the electrons. Accordingly, when oneswitch (e.g., the first switch 408) is switched “on” (i.e., theelectrons drift towards the p-doped region 428), the other switch (e.g.,the second switch 410) is switched “off” (i.e. the electrons are blockedfrom the p-doped region 438). In some implementations, the first controlsignal 422 and the second control signal 432 may be voltages that aredifferential to each other.

In general, a difference (before equilibrium) between the Fermi level ofa p-doped region and the Fermi level of an n-doped region creates anelectric field between the two regions. In the first switch 408, anelectric field is created between the p-doped region 428 and the n-dopedregion 426. Similarly, in the second switch 410, an electric field iscreated between the p-doped region 438 and the n-doped region 436. Whenthe first switch 408 is switched “on” and the second switch 410 isswitched “off”, the electrons drift toward the p-doped region 428, andthe electric field between the p-doped region 428 and the n-doped region426 further carries the electrons to the n-doped region 426. The readoutcircuit 424 may then be enabled to process the charges collected by then-doped region 426. On the other hand, when the second switch 410 isswitched “on” and the first switch 408 is switched “off”, the electronsdrift toward the p-doped region 438, and the electric field between thep-doped region 438 and the n-doped region 436 further carries theelectrons to the n-doped region 436. The readout circuit 434 may then beenabled to process the charges collected by the n-doped region 436.

In some implementations, the substrate 402 may be coupled to an externalcontrol 416. For example, the substrate 402 may be coupled to anelectrical ground, or a preset voltage less than the voltages at then-doped regions 426 and 436. In some other implementations, thesubstrate 402 may be floated and not coupled to any external control.

FIG. 4B is an example switched photodetector 450 for converting anoptical signal to an electrical signal. The switched photodetector 450is similar to the switched photodetector 400 in FIG. 4A, but that thefirst switch 408 and the second switch 410 further includes an n-wellregion 452 and an n-well region 454, respectively. In addition, theabsorption layer 406 may be a p-doped layer and the substrate 402 may bea p-doped substrate. In some implementations, the doping level of then-well regions 452 and 454 may range from 10¹⁵ cm⁻³ to 10¹⁷ cm⁻³. Thedoping level of the absorption layer 406 and the substrate 402 may rangefrom 10¹⁴ cm⁻³ to 10¹⁶ cm⁻³.

The arrangement of the p-doped region 428, the n-well region 452, theabsorption layer 406, the n-well region 454, and the p-doped region 438forms a PNPNP junction structure. In general, the PNPNP junctionstructure reduces a leakage current from the first control signal 422 tothe second control signal 432, or alternatively from the second controlsignal 432 to the first control signal 422.

The arrangement of the n-doped region 426, the p-doped substrate 402,and the n-doped region 436 forms an NPN junction structure. In general,the NPN junction structure reduces a charge coupling from the firstreadout circuit 424 to the second readout circuit 434, or alternativelyfrom the second readout circuit 434 to the first readout circuit 424.

In some implementations, the p-doped region 428 is formed entirelywithin the n-well region 452. In some other implementations, the p-dopedregion 428 is partially formed in the n-well region 452. For example, aportion of the p-doped region 428 may be formed by implanting thep-dopants in the n-well region 452, while another portion of the p-dopedregion 428 may be formed by implanting the p-dopants in the absorptionlayer 406. Similarly, in some implementations, the p-doped region 438 isformed entirely within the n-well region 454. In some otherimplementations, the p-doped region 438 is partially formed in then-well region 454. In some implementations, the depth of the n-wellregions 452 and 454 is shallower than the p-doped regions 428 and 438.

FIG. 4C is an example switched photodetector 460 for converting anoptical signal to an electrical signal. The switched photodetector 460is similar to the switched photodetector 400 in FIG. 4A, but that theabsorption layer 406 further includes an n-well region 456. In addition,the absorption layer 406 may be a p-doped region and the substrate 402may be a p-doped substrate. In some implementations, the doping level ofthe n-well region 456 may range from 10¹⁵ cm⁻³ to 10¹⁷ cm⁻³. The dopinglevel of the absorption layer 406 and the substrate 402 may range from10¹⁴ cm⁻³ to 10¹⁶ cm⁻³.

The arrangement of the p-doped region 428, the n-well region 456, andthe p-doped region 438 forms a PNP junction structure. In general, thePNP junction structure reduces a leakage current from the first controlsignal 422 to the second control signal 432, or alternatively from thesecond control signal 432 to the first control signal 422.

The arrangement of the n-doped region 426, the p-doped absorption layer406, and the n-doped region 436 forms an NPN junction structure. Ingeneral, the NPN junction structure reduces a charge coupling from thefirst readout circuit 424 to the second readout circuit 434, oralternatively from the second readout circuit 434 to the first readoutcircuit 424.

In some implementations, the p-doped regions 428 and 438 are formedentirely within the n-well region 456. In some other implementations,the p-doped regions 428 and 438 are partially formed in the n-wellregion 456. For example, a portion of the p-doped region 428 may beformed by implanting the p-dopants in the n-well region 456, whileanother portion of the p-doped region 428 may be formed by implantingthe p-dopants in the absorption layer 406. In some implementations, thedepth of the n-well region 456 is shallower than the p-doped regions 428and 438.

FIG. 4D is an example switched photodetector 470 for converting anoptical signal to an electrical signal. The switched photodetector 470is similar to the switched photodetector 460 in FIG. 4C, but that then-well region 458 is formed to extend from the absorption layer 406 intothe substrate 202. In addition, the absorption layer 406 may be ap-doped region and the substrate 402 may be a p-doped substrate. In someimplementations, the doping level of the n-well region 456 may rangefrom 10¹⁵ cm⁻³ to 10¹⁷ cm⁻³. The doping level of the absorption layer406 and the substrate 402 may range from 10¹⁴ cm⁻³ to 10¹⁶ cm⁻³.

The arrangement of the p-doped region 428, the n-well region 458, andthe p-doped region 438 forms a PNP junction structure, which furtherreduces a leakage current from the first control signal 422 to thesecond control signal 432, or alternatively from the second controlsignal 432 to the first control signal 422. The arrangement of then-doped region 426, the p-doped substrate 402, the n-well region 458,the p-doped substrate 402, and the n-doped region 436 forms an NPNPNjunction structure, which further reduces a charge coupling from thefirst readout circuit 424 to the second readout circuit 434, oralternatively from the second readout circuit 434 to the first readoutcircuit 424. In some implementations, the n-well region 458 alsoeffectively reduces the potential energy barrier perceived by theelectrons flowing from the absorption layer 406 to the substrate 402.

FIG. 4E is an example switched photodetector 480 for converting anoptical signal to an electrical signal. The switched photodetector 480is similar to the switched photodetector 400 in FIG. 4A, but that theswitched photodetector 480 further includes one or more p-well regions446 and one or more p-well regions 448. In some implementations, the oneor more p-well regions 446 and the one or more p-well regions 448 may bepart of a ring structure that surrounds the first switch 408 and thesecond switch 410. In some implementations, the doping level of the oneor more p-well regions 446 and 448 may range from 10¹⁵ cm⁻³ to 10²⁰cm⁻³. The one or more p-well regions 446 and 448 may be used as anisolation of photo-electrons from the neighboring pixels.

Although not shown in FIG. 4A-4E, in some implementations, an opticalsignal may reach to the switched photodetector from the backside of thesubstrate 402. One or more optical components (e.g., microlens orlightguide) may be fabricated on the backside of the substrate 402 tofocus, collimate, defocus, filter, or otherwise manipulate the opticalsignal.

Although not shown in FIG. 4A-4E, in some other implementations, thefirst switch 408 and the second switch 410 may alternatively befabricated to collect holes instead of electrons. In this case, thep-doped region 428 and the p-doped region 438 would be replaced byn-doped regions, and the n-doped region 426 and the n-doped region 436would be replaced by p-doped regions. The n-well regions 452, 454, 456,and 458 would be replaced by p-well regions. The p-well regions 446 and448 would be replaced by n-well regions.

Although not shown in FIG. 4A-4E, in some implementations, theabsorption layer 406 may be bonded to a substrate after the fabricationof the switched photodetector 400, 450, 460, 470, and 480. The substratemay be any material that allows the transmission of the optical signal412 to reach to the switched photodetector. For example, the substratemay be polymer or glass. In some implementations, one or more opticalcomponents (e.g., microlens or lightguide) may be fabricated on thecarrier substrate to focus, collimate, defocus, filter, or otherwisemanipulate the optical signal 412.

Although not shown in FIGS. 4A-4E, in some implementations, the switchedphotodetector 400, 450, 460, 470, and 480 may be bonded (ex: metal tometal bonding, oxide to oxide bonding, hybrid bonding) to a secondsubstrate with circuits including control signals, and/or, readoutcircuits, and/or phase lock loop (PLL), and/or analog to digitalconverter (ADC). A metal layer may be deposited on top of the switchedphotodetector that may be used as a reflector to reflect the opticalsignal incident from the backside of the substrate 402. Adding such amirror like metal layer may increase the absorption efficiency (quantumefficiency) of the absorption layer 406. For example, the absorptionefficiency of photodetectors operating at a longer NIR wavelengthbetween 1.0 μm and 1.6 μm may be significantly improved by addition of areflecting metal layer. An oxide layer may be included between the metallayer and the absorptive layer to increase the reflectivity. The metallayer may also be used as the bonding layer for the wafer-bondingprocess. In some implementations, one or more switches similar to 408and 410 can be added for interfacing control signals/readout circuits.

Although not shown in FIG. 4A-4E, in some implementations, theabsorption layer 406 may be partially or fully embedded/recessed in thesubstrate 402 to relieve the surface topography and so ease thefabrication process. An example of the embedment technique is describedin U.S. Patent Publication No. US20170040362 A1.

FIG. 5A shows an example photodetector 500 for converting an opticalsignal to an electrical signal. The photodetector 500 includes anabsorption layer 506 fabricated on a substrate 502, and a first layer508 formed on top of the absorption layer 506 and the substrate 502. Thesubstrates 502 may be similar to the substrate 102 described previously,and the absorption layers 506 may be similar to the absorption layer 106described previously, and may be formed, for example, from Ge or GeSiwith Ge concentration ranging from 1-99%. The background doping polarityand doping level of the Ge or GeSi absorption layer 506 may be p-typeand range from 10¹⁴ cm⁻³ to 10¹⁶ cm⁻³. The background doping level maybe due to, for example, explicit incorporation of doping, or due tomaterial defects introduced during formation of the absorption layer506. The absorption layer 506 of the photodetector 500 has a mesastructure and is supported by the substrate 502, and while a verticalsidewall has been shown, the shape and sidewall profile of the mesastructure may depend on the specifics of the growth and fabricationprocess for the absorption layer 506.

The first layer 508 covers an upper surface and side surfaces of theabsorption layer 506, and covers a portion of an upper surface of thesubstrate 502 on which the absorption layer 506 is formed. The firstlayer 508 may be formed from a Complementary Metal-Oxide-Semiconductor(CMOS) process compatible material (CPCM), such as amorphous silicon,polysilicon, epitaxial silicon, aluminum oxide family (e.g., Al₂O₃),silicon oxide family (e.g., SiO₂), Ge oxide family (e.g., GeO₂),germanium-silicon family (e.g., Ge_(0.4)Si_(0.6)), silicon nitridefamily (e.g., Si₃N₄), high-k materials (e.g. HfOx, ZnOx, LaOx, LaSiOx),and any combination thereof. The presence of the first layer 508 overthe surfaces of the absorption layer 506 may have various effects. Forexample, the first layer 508 may act as a surface passivation layer tothe absorption layer 506, which may reduce dark current or leakagecurrent generated by defects present at the surface of the absorptionlayer 506. In the case of a germanium (Ge) or a germanium-silicon (GeSi)absorption layer 506, the surface defects may be a significant source ofdark current or leakage current, which contributes to an increased noiselevel of the photocurrent generated by the photodetector 500. By formingthe first layer 508 over the surfaces of the absorption layer 506, thedark current or leakage current may be reduced, thereby reducing thenoise level of the photodetector 500. As another example, the firstlayer 508 may modulate a Schottky barrier level between a contact formedon the photodetector 500 and the absorption layer 506 and/or thesubstrate 502. This barrier modulation effect will be described at alater paragraph.

FIG. 5B shows an example photodetector 510 for converting an opticalsignal to an electrical signal. The photodetector 510 is similar to thephotodetector 500 in FIG. 5A, but differs in that the absorption layer506 is partially embedded in a recess formed on the substrate 502, andthe photodetector 510 further includes spacers 512. The spacers 512 maybe a dielectric material such as various oxides and nitrides thatseparates the sidewalls of the absorption layer 506 from the substrate502. In some implementations, the spaces 512 may be omitted, and theembedded portion of the absorption layer 506 may be in direct contactwith a surface of the recess formed in the substrate 502, such as a[110] sidewall of a silicon substrate. An example of the embedmenttechnique is described in U.S. Patent Publication No. US20170040362 A1.

FIG. 5C shows an example photodetector 520 for converting an opticalsignal to an electrical signal. The photodetector 520 is similar to thephotodetector 510 in FIG. 5B, but differs in that the absorption layer506 is fully embedded in the recess formed on the substrate 502. Anexample of the embedment technique is described in U.S. PatentPublication No. US20170040362 A1.

FIG. 5D shows an example switched photodetector 530 for converting anoptical signal to an electrical signal. The switched photodetector 530is similar to the photodetector 510 in FIG. 5B, but differs in that afirst switch 532 and a second switch 542 have been fabricated in theabsorption layer 506 and the first layer 508. The first switch 532 maybe similar to the first switch 108 in FIG. 1A, but further includes afirst readout contact 535 coupled to a first n-doped region 534 and afirst control contact 538 coupled to a first p-doped region 537.Similarly, the second switch 542 may be similar to the second switch 110in FIG. 1A, but further includes a second readout contact 545 coupled toa second n-doped region 544 and a second control contact 548 coupled toa second p-doped region 547. The first and second p-doped regions 537and 547 may be control regions, and the first and second n-doped regions534 and 544 may be readout regions. The first and second readoutcontacts 535 and 545 are connected to respective readout circuitssimilar to readout circuits 124 and 134 shown in FIG. 1A. The first andsecond control contacts 538 and 548 are connected to respective controlsignals, such as the control signals 122 and 132 shown in FIG. 1A.

The contacts 535, 538, 545, and 548 provide electrical contacts to therespective doped regions, and may be formed from various electricallyconducting materials. Examples of contact materials include varioussilicides, Ta—TaN—Cu stack, Ti—TiN—W stack, aluminum, and variouscombinations of such materials. In some implementations, the readoutcontacts 535 and 545 may be formed from different materials than thecontrol contacts 538 and 548. The contacts 535, 538, 545, and 548 mayhave various physical configurations. The dimensions of the contacts mayrange from being as small as 10's of nanometers in diameter or width.While a single contact 535, 538, 545, or 548 are shown to be coupled toeach of the doped regions, two or more contacts may be coupled to thedoped regions to, for example, reduce contact resistance or improvereliability, as is customary in semiconductor device manufacturingprocess.

FIG. 5E shows an example switched photodetector 550 for converting anoptical signal to an electrical signal. The switched photodetector 550is similar to the switched photodetector 530 in FIG. 5D, but differs inthat the first switch 532 and the second switch 542 further includen-well regions 539 and 549, respectively, and p-well regions 536 and546, respectively. Additions of the n-well regions and the p-wellregions may modify the electrical and/or optical properties of thephotodetector 550. In some implementations, the doping level of then-well regions 539 and 549 and p-well regions 536 and 546 may range from10¹⁵ cm⁻³ to 10¹⁷ cm⁻³.

The arrangement of the p-doped region 537, the n-well region 539, ap-type absorption layer 506, the n-well region 549, and the p-dopedregion 547 forms a PNPNP junction structure. In general, the PNPNPjunction structure reduces a flow of leakage current from the firstcontrol signal 122 to the second control signal 132, or alternativelyfrom the second control signal 132 to the first control signal 122. Thearrangement of the n-doped region 534, the p-well region 536, the p-typeabsorption layer 506, the p-well region 546, and the n-doped region 544forms an NPN junction structure. In general, the NPN junction structurereduces a charge coupling from the first readout circuit 124 to thesecond readout circuit 134, or alternatively from the second readoutcircuit 134 to the first readout circuit 124.

In some implementations, the p-doped region 537 is formed entirelywithin the n-well region 539. In some other implementations, the p-dopedregion 537 is partially formed in the n-well region 539. For example, aportion of the p-doped region 537 may be formed by implanting thep-dopants in the n-well region 539, while another portion of the p-dopedregion 537 may be formed by implanting the p-dopants in the absorptionlayer 506. Similarly, in some implementations, the p-doped region 547 isformed entirely within the n-well region 549. In some otherimplementations, the p-doped region 547 is partially formed in then-well region 549. In some implementations, the n-well regions 539 and549 form a continuous n-well region that includes at least a portion ofboth the p-doped regions 537 and 547.

In some implementations, the n-doped region 534 is formed entirelyoutside the p-well region 536. In some other implementations, then-doped region 534 is partially formed in the p-well region 536. Forexample, a portion of the n-doped region 534 may be formed by implantingthe n-dopants in the p-well region 536, while another portion of then-doped region 534 may be formed by implanting the n-dopants in theabsorption layer 506. Similarly, in some implementations, the n-dopedregion 544 is formed entirely outside the p-well region 546. In someother implementations, the n-doped region 544 is partially formed in thep-well region 546.

While FIGS. 5D and 5E show switched photodetectors with a partiallyembedded absorption region 506, the same construction can be applied tophotodetector 500 having non-embedded absorption layer 506, and tophotodetector 520 having a fully-embedded absorption layer 506 toachieve analogous effects.

While n-well regions 539 and 549, and p-well regions 536 and 546 havebeen illustrated in combination for the purpose of illustration, thewells may be individually implemented, or implemented in anycombination.

FIG. 5F shows an example switched photodetector 560 for converting anoptical signal to an electrical signal. The switched photodetector 560is similar to the switched photodetector 530 in FIG. 5D, but differs inthat the respective p-doped regions 537 and 547 of switches 532 and 542have been omitted. As a result, the first and second control contacts538 and 548 form Schottky junctions to the first layer 508. Schottkyjunction is an electrical junction formed between a metal and asemiconductor, when the semiconductor is not intentionally doped ordoped to a moderate dopant concentration, such as below approximately1×10¹⁵ cm⁻³. A region 562 marks a leakage path between the first controlcontact 538 and the second control contact 548 through the first layer508 and the absorption layer 506, which will be described in more detailin relation to FIG. 5G.

FIG. 5G shows an example band diagram 570 of the leakage path formedbetween the control contacts 538 and 548. The band diagram 570illustrates various energy levels that charge carriers such as anelectron 572 and a hole 574 experiences at various locations along aleakage path. The vertical axis corresponds to an energy level E, andthe horizontal axis corresponds to a position x along the leakage pathformed between the control contacts 538 and 548. An example scenariowhere the electrical potential energy of the first control contact 538is higher than that of the second control contact 548 (e.g., firstcontrol signal 122 has a lower voltage than the second control signal132) is shown. The potential difference manifests as a downward slope ofthe overall band diagram from the first control contact 538 to thesecond control contact 548. The energy levels and positions as shown arefor illustration purposes, and may not represent actual values.

An electron barrier 573 and a hole barrier 575 are examples of aSchottky barrier. A Schottky junction is characterized by presence of aSchottky barrier, which is a potential energy barrier that needs to beovercome by the electron 572 and hole 574 in order for those carriers toflow across the Schottky junction. The value of the barriers 573 and 575can vary depending on a work function of the material of contacts 538and 548, and the material of the first layer 508. As such, by selectingan appropriate combination of contact material and first layer material,a desired level of electron barrier 573 and hole barrier 575 may be set.

The electron 572 must overcome the electron barrier 573 between thefirst control contact 538 and the first layer 508. By providing asufficiently high electron barrier 573, the voltage potential of thecontrol signal 122 applied to the first control contact may be unable toovercome the barrier 573. As such, the electron barrier 573 may blockthe electron 572 from flowing into the absorption layer 506. In caseswhere the electron 572 does overcome the electron barrier 573, which maybe due to statistical fluctuation of a thermal energy of the electron572 (“thermionic emission”) or quantum tunneling, the electron 572 mayflow across the absorption layer 506 to the first layer 508 adjacent tothe second control contact 548. Another electron barrier is presented bya junction formed between the absorption layer 506 and the first layer508, which may further block electron 572 from flowing into the secondcontrol contact 548, thereby reducing a leakage current of electronsfrom the first control contact 538 to the second control contact 548.

Similarly, the hole 574 must overcome the hole barrier 575 between thesecond control contact 548 and the first layer 508. By providing asufficiently high hole barrier 575, the voltage potential of the controlsignal 132 applied to the second control contact may be unable toovercome the barrier 575. As such, the hole barrier 575 may block thehole 574 from flowing into the absorption layer 506. In cases where thehole 574 does overcome the hole barrier 575, which may be due tostatistical fluctuation of a thermal energy of the hole 574 (“thermionicemission”) or quantum tunneling, the hole 574 may flow across theabsorption layer 506 to the first layer 508 adjacent to the firstcontrol contact 538. Another hole barrier is presented by a junctionformed between the absorption layer 506 and the first layer 508, whichmay further block hole 574 from flowing into the first control contact538, thereby reducing a leakage current of holes from the second controlcontact 548 to the first control contact 538.

When light is being illuminated to the absorption layer 506, the photon576 of the light may be absorbed by an electron in a valence band of theabsorption layer 506 and, resulting in creation of an electron-hole asindicated by the vertical arrow adjacent to the photon 576. The electronof this electron-hole pair forms a photocurrent that is to be capturedby the readout circuits 124 and/or 134 through the respective readoutcontacts 535 and/or 545, and should not flow into the control contacts538 and 548. In this case, the barriers formed by the interface betweenthe first layer 508 and the absorption layer 506 may prevent such aflow, thereby improving photocurrent collection efficiency of thereadout circuits.

When the first layer 508, such as amorphous silicon or polysilicon orcrystalline silicon or germanium-silicon, is inserted between thecontrol contacts 538 and 548 and the absorption layer 506, such as aGeSi mesa, the Schottky barrier of the Metal-Semiconductor (MS) junctionis modified, resulting in partial blocking of the electrons or holesinjected into the first layer 508 by the contacts 538 and 548 asexplained above. The power consumption of a ToF pixel such as theswitched photodetectors described herein is partially determined by aleakage current flowing between the two control contacts 538 and 548connected to the two control circuits. As such, by partially blockingthe electrons or holes injected by the contacts 538 and 548, the powerconsumption of the ToF pixel can be significantly reduced.

FIG. 5H shows an example switched photodetector 580 for converting anoptical signal to an electrical signal. The switched photodetector 580is similar to the switched photodetector 560 in FIG. 5F, but differs inthat the photodetector 580 further includes the n-well regions 539 and549, and the p-well regions 536 and 546. The structures and effects ofthe n-well regions 539 and 549, and the p-well regions 536 and 546 havebeen described in relation to FIG. 5E. In addition, the n-well regions539 and 549 overlap with at least a portion of the first layer 508beneath the control contacts 538 and 548, which can contribute to anenhancement in the voltage drop inside the absorption layer 506.Enhancement in the voltage drop inside the absorption layer 506increases the magnitude of electric field established within theabsorption layer 506, which may improve capturing efficiency of thephoto-generated electrons by the readout circuits 124 and/or 134 throughthe respective readout contacts 535 and/or 545.

FIG. 5I shows an example switched photodetector 582 for converting anoptical signal to an electrical signal. The switched photodetector 582is similar to the switched photodetector 550 in FIG. 5E, but differs inthat the first switch 532 is now located on the substrate 502 andadjacent to the absorption region 506 on the left side, and the secondswitch 542 are now located on the substrate 502 and adjacent to theabsorption region 506 on the right side. The operations of the switchedphotodetector 582 is analogous to that of previously described switchedphotodetectors. However, as electrical contacts formed between contactssuch as readout contacts 535 and 545 or control contacts 538 and 548 andsilicon substrate 502 generally have a lower dark current or leakagecurrent than electrical contacts formed between contacts and Ge or GeSiabsorption layer 506 (e.g., due to the substrate 502 having lessmaterial defects compared to the absorption layer 506), the overall darkcurrent or leakage current may be lowered in comparison to theconfiguration of photodetector 550 shown in FIG. 5E. Further, as aresult of the switches being placed on the substrate 502, thephoto-generated carriers from the light absorbed by the absorptionregion 506 now flows from the absorption region 506 to the substrate 502before reaching the readout circuits 124 and 134. Depending on thespecific geometry of the absorption region 506 and the spacer 512 andtheir material, the photo-carriers may conduct through the spacer 512,flow around the spacer 512, or combination thereof.

In some implementations, the p-doped regions 537 and 547 may be omittedin a configuration analogous to the configuration shown in FIG. 5F.While n-well regions 539 and 549, and p-well regions 536 and 546 havebeen illustrated in combination for the purpose of illustration, thewells may be omitted, be individually implemented, or implemented in anycombination.

FIG. 5J shows an example switched photodetector 586 for converting anoptical signal to an electrical signal. The switched photodetector 586is similar to the switched photodetector 582 in FIG. 5I, but differs inthat the respective p-doped regions 537 and 547 of switches 532 and 542have been omitted. As a result, the first and second control contacts538 and 548 form Schottky junctions to the first layer 508. The effectsof the Schottky junctions have been described in relation to FIGS. 5F-H.The band diagram 570 of FIG. 5G remain applicable to the region 562 ofthe photodetector 586, with the barriers formed by to the first layer508 now corresponding to barriers formed by the first layer 508, thesubstrate 502, and the spacer 512 due to the modified geometry ofphotodetector 586 relative to photodetector 506.

While the n-well regions 539 and 549, and the p-well regions 536 and 546have been illustrated in combination for the purpose of illustration,the wells may be omitted, be individually implemented, or implemented inany combination.

FIG. 5K shows an example switched photodetector 588 for converting anoptical signal to an electrical signal. The switched photodetector 588is similar to the switched photodetector 582 in FIG. 5I, but differs inthat the first switch 532 further includes a second p-doped region 537a, a third control contact 538 a coupled to the second p-doped region537 a, and a second n-well region 539 a in contact with the secondp-doped region 537 a, and the second switch 542 further includes asecond p-doped region 547 a, a fourth control contact 548 a coupled tothe second p-doped region 547 a, and a second n-well region 549 a incontact with the second p-doped region 547 a. The second p-doped regions537 a and 537 b are similar to second p-doped regions 537 and 547,respectively. The second n-well regions 539 a and 549 a are similar tosecond n-well regions 539 and 549, respectively. The third controlcontact 538 a is similar to the first control contact 538, and thefourth control contact 548 a is similar to the second control contact548. The third control contact 538 a is connected to the first controlsignal 122, and the fourth control contact 548 a is connected to thesecond control signal 132.

As the first control contact 538 and the associated doped regions arenot in direct contact with the absorption region 506, the electric fieldgenerated within the absorption region 506 by application of the firstcontrol signal 122 to the first control contact 538 may be relativelyweak in comparison to a configuration where the first control contact538 is in direct contact with the absorption layer 506, such as in theconfiguration of the photodetector 550 in FIG. 5E. By adding the thirdand fourth control contacts 538 a and 548 a and associated dopedregions, the carrier collection control efficiency of the photodetector586 may be improved over that of the photodetector 582 of FIG. 5I to becomparable to the carrier collection control efficiency of thephotodetector 550 in FIG. 5E, while at least partially retaining thebenefit of reduced dark current or leakage current by moving thecontacts to the substrate 502. Furthermore, as larger electric fieldwithin the absorption region can lead to increased photodetectorbandwidth, faster switching between the first switch 532 and the secondswitch 542, the additional control contacts 538 a and 548 a may alsocontribute to an improvement in operational speed of the photodetector584.

While the third control contact 538 a and the fourth control contact 548a are shown to share the respective control signal 122 and 132 for thefirst control contact 538 and the second control contact 548, in someimplementations, the contacts 538 a and 548 a may have independentcontrol signals that may be different from first and second controlsignals 122 and 132. For example, the control signal for the thirdcontrol contact 538 a may be smaller than the first control signal 122for the first control contact 538, as the control signal applied to thethird control contact 538 a may be have a greater effect on thephoto-generated carriers than the first control signal 122 applied tothe first control contact 538 due to the proximity of the second p-dopedregion 537 a to the carriers being generated in the absorption region506, and the same applies to the control signal for the fourth controlcontact 548 a.

In some implementations, the second p-doped regions 537 a and 547 a maybe omitted to form Schottky junctions, the effects of which have beenpreviously described in relation to FIGS. 5F-5H. While the n-wellregions 539 and 549, and the p-well regions 536 and 546 have beenillustrated in combination for the purpose of illustration, the wellsmay be omitted, be individually implemented, or implemented in anycombination.

While various configurations of the switched photodetectors having apartially embedded absorption layer 506 have been described in FIGS.5D-5K, the described configurations can be applied to switchedphotodetectors having a fully protruding absorption layer 506 such asthe configuration shown in FIG. 5A, and to switched photodetectorshaving a fully embedded absorption layer 506 such as the configurationshown in FIG. 5C and achieve analogous effects.

The photodetectors described in FIGS. 5A-5K may be incorporated into afront-side illumination (FSI) image sensor, or a back-side illuminationimage sensor (BSI). In the FSI configuration, the light enters thephotodetectors from the top through the first layer 508. In the BSIconfiguration, the light enters the photodetectors from the bottomthrough the substrate 502.

The control regions (e.g., p-doped regions 537 and 547) and the readoutregions (e.g., n-doped regions 534 and 544) may be at different heights.For example, in the case of photodetectors 530, 550, 560, and 580, andany configurations in which the control regions and the readout regionsare both located on the absorption region 506, a portion of theabsorption region 506 corresponding to the readout region or the controlregion may be etched, and the readout region or the control region maybe formed on the etched portion, resulting in a vertical offset betweenthe control region and the readout region. Similarly, in the case ofphotodetectors 582, 586, and 588, and any configurations in which thecontrol regions and the readout regions are both located on thesubstrate 502, a portion of the substrate 502 corresponding to thereadout region or the control region may be etched, and the readoutregion or the control region may be formed on the etched portion,resulting in a vertical offset between the control region and thereadout region

In some implementations, lens may be placed on an optical path of lightincident on the photodetectors. The lens may be, for example, a microball lens or a Fresnel Zone Plate (FZP) lens. As another example, for asilicon substrate 502, the lens may be formed directly on the substrate502 by etching of the substrate 502. Details regarding configurations ofthe lens will be provided in relation to FIGS. 7A-7C.

In some implementations, the interface between the absorption layer 506and the spacers 512 may be doped with n- or p-type dopants to improveelectrical isolation for holes and electrons, respectively. In someimplementations, the interface between the absorption layer 506 and thesubstrate 502 (e.g., the bottom interface) may be doped with n- orp-type dopants to improve electrical isolation for holes and electrons,respectively.

FIG. 6A shows an example switched photodetector 600 for converting anoptical signal to an electrical signal. The switched photodetector 600includes the substrate 502, the absorption region 506, the first switch532, the second switch 542, and a counter-doped region 610. Thecounter-doped region 610 is arranged within the absorption region 506.The first and second switches 532 and 542 are arranged on the absorptionlayer 506. The substrate 502, the absorption region 506, and first andsecond switches 532 and 542 have been previously described in relationto FIG. 5D.

The counter-doped region 610 is a portion of the absorption region 506that has been doped with a dopant specie to reduce a net carrierconcentration of the absorption region 506. An undoped semiconductormaterial has a certain concentration of charge carriers that maycontribute to current conduction even in absence of dopants, which isreferred to as the intrinsic carrier concentration of the semiconductor.The absorption region 506 is typically formed from semiconductormaterials, such as Silicon, Germanium, or an alloy of the two, and hasan associated intrinsic carrier concentration. This intrinsic carrierconcentration may vary depending on various factors, such as thematerial preparation method and defect level (defect concentration).Examples of the material preparation method include epitaxial growth,chemical vapor deposition (CVD), metal organic CVD (MOCVD), and physicalvapor deposition (PVD), and materials prepared using different methodsmay be different material defect levels. Typically, higher number ofmaterial defects correlates to higher level of intrinsic carrierconcentration level. For example, bulk crystalline Germanium may have anintrinsic p-type like carrier concentration of approximately 2*10¹³ cm⁻³at room temperature, while an epitaxially grown Germanium may have anintrinsic p-type like carrier concentration that is an order ofmagnitude higher at approximately 5*10¹⁴ cm⁻³. Depending on the materialtype and the nature of the defects, the semiconductor material may bep-type or n-type like.

Reducing a leakage current of switched photodetectors, such as thephotodetector 600, is important for reducing a power consumption of aTime-Of-Flight pixel. One of the contributors to the leakage current ofswitched photodetectors is a leakage current flowing between the controlregions, e.g. the current flow between the p-doped regions 537 and 547.One approach to reducing such current flow is by reducing a net carrierconcentration of the absorption region 506 between the two p-dopedregions 537 and 547. The net carrier concentration is the concentrationof carriers available for conducting the current, and may be determinedby combining the contributions of the intrinsic carrier concentrationwith extrinsic carrier concentration contributed by the dopants. Byappropriately selecting the electrical type, species, and concentrationof the dopants, the intrinsic carrier concentration may be compensated,or “counter-doped,” by the dopants, resulting in a lower net carrierconcentration for the semiconductor material. Typically, the leakagecurrent between the control regions is proportional to the net carrierconcentration when the intrinsic and net carriers have the samepolarity, i.e., both are p-type like or n-type like.

The type of dopants to be used for the counter-doped region 610 may beselected based on various factors, such as the material forming theabsorption region 506 and the nature of defects present in theabsorption region 506. For example, epitaxially grown Ge on Si substrate502 is typically a p-type material. In such a case, an n-type dopantspecie such a P, As, Sb, or F may be used to dope the counter-dopedregion 610. The doping may be performed in various ways, includingimplantation, diffusion, and in-situ doping during growth of thematerial. In some cases, dopants such as fluorine may passivate thedefects. The passivated defects may stop acting as sources of chargecarriers and therefore the net carrier concentration of theFluorine-doped absorption region 506 may be reduced and become moreintrinsic.

The concentration of dopants to be used for the counter-doped region 610may be selected based on the intrinsic carrier concentration of theabsorption region 506. For example, an epitaxially grown Germaniumhaving an intrinsic carrier concentration of approximately 5*10¹⁴ cm⁻³may be doped with a counter-dopant concentration of approximately 5*10¹⁴cm⁻³ to reduce the intrinsic carrier concentration toward that of thebulk crystalline Ge of approximately 2*10¹³ cm⁻³. In general, thecounter-doping concentration may range from 1*10¹³ cm⁻³ to 1*10¹⁶ cm⁻³.In some implementations, the counter-doped region 610 may have variabledopant concentrations across the region. For example, regions that arecloser to material interfaces, such as the bottom of the absorber 506,may have a higher intrinsic carrier concentration due to increaseddefect level, which may be better compensated by a correspondingly highcounter-doping level. In some implementations, the counter-dopantconcentration may be greater than the intrinsic carrier concentration ofthe absorption region 506. In such cases, the polarity of the absorptionregion 506 may be changed from p-type to n-type or vice versa.

While the counter-doped region 610 is shown to completely cover then-doped regions 534 and 544, and the p-doped regions 537 and 547, ingeneral, the counter-doped region 610 may cover just the p-doped regions537 and 547 or the n-doped regions 534 and 544. Additionally, while thecounter-doped region 610 is shown to be a continuous region, in general,it may be two or more separate regions. Furthermore, while thecounter-doped region 610 is shown to be only a portion of the absorptionregion 506, in general, the counter-doped region 610 may be formedacross the entire absorption region 506.

In some implementations, the counter-doped region 610 may function as adopant diffusion suppressor, which may contribute to formation of anabrupt junction profile. Formation of an abrupt junction profile betweenthe counter-doped region 610 and the p-doped regions 537 and 547 maylead to a lower leakage current and thereby reduce the power consumptionof ToF pixels. For example, in the case of a Ge absorption region 506,Fluorine doping may suppress diffusion of Phosphorous dopants in then-doped region 534.

In general, the counter-doped region 610 may be implemented in variousimplementations of the switched photodetectors to reduce the leakagecurrent between control regions.

In some implementations, the p-doped regions 537 and 547 may be omitted,resulting in formation of Schottky junctions, the effects of which havebeen described in relation to FIGS. 5F-5H.

FIG. 6B shows an example switched photodetector 620 for converting anoptical signal to an electrical signal. The switched photodetector 620is similar to the photodetector 600 in FIG. 6A but differs in that thefirst switch 532 and the second switch 542 further include n-wellregions 612 and 614, respectively. Additions of the n-well regions maymodify the electrical and/or optical properties of the photodetector620. In some implementations, the doping level of the n-well regions 612and 614 may range from 10¹⁵ cm⁻³ to 10¹⁷ cm⁻³. In some implementations,the n-well regions 612 and 614 may extend from the upper surface of theabsorption region 506 to the lower surface of the counter-doped region610 or to the interface between the absorption layer 506 and thesubstrate 502.

The arrangement of the p-doped region 537, the n-well region 612, thecounter-doped region 610, the n-well region 614, and the p-doped region547 forms a PNINP junction structure. In general, the PNINP junctionstructure reduces a flow of leakage current from the first controlsignal 122 to the second control signal 132, or alternatively from thesecond control signal 132 to the first control signal 122.

In some implementations, the p-doped region 537 is formed entirelywithin the n-well region 612. In some other implementations, the p-dopedregion 537 is partially formed in the n-well region 612. For example, aportion of the p-doped region 537 may be formed by implanting thep-dopants in the n-well region 612, while another portion of the p-dopedregion 537 may be formed by implanting the p-dopants in thecounter-doped region 610. Similarly, in some implementations, thep-doped region 547 is formed entirely within the n-well region 614. Insome other implementations, the p-doped region 547 is partially formedin the n-well region 614. In some implementations, the n-well regions612 and 614 form a continuous n-well region that includes at least aportion of both the p-doped regions 537 and 547.

Operation speed or bandwidth of a photodetector can be an importantperformance parameter for applications that benefit from high speeddetection of light, such as ToF detection. Among characteristics thatcan affect bandwidth of a photodetector is the physical size of thephotodetector, such as the area of the photodetector through which lightis received. Reducing the area of the photodetector, for example, canlead to a reduction in device capacitance, carrier transit time, or acombination of both, which typically results in an increase inphotodetector bandwidth. However, a reduction in the detection area of aphotodetector can lead to a reduction in the amount of light (i.e.,number of photons) detected by the photodetector. For example, for agiven intensity of light per unit area, the reduction in the area of thedetector leads to a reduction in detected light.

For applications that benefit from both high bandwidth and highdetection efficiency, such as ToF detection, addition of a microlensbefore the photodetector may be beneficial. The microlens can focus theincident light onto the photodetector, allowing a small-areaphotodetector to detect light incident over an area larger than itself.For example, a properly designed combination of a microlens and a spacerlayer (SL) that separates the microlens from the photodetector by aneffective focal length of the microlens can allow focusing of theincident light to a diffraction-limited spot that is on the order of thesquare of the optical wavelength of the incident light. Such a schemecan allow reduction of photodetector area while mitigating the potentialdownsides of the photodetector area reduction.

FIG. 7A shows a cross-sectional view of an example configuration 700 ofsilicon lenses integrated with photodetectors. The configuration 700includes a donor wafer 710 and a carrier wafer 730. The donor wafer 710includes multiple pixels 720 a through 720 c (collectively referred toas pixels 720), via 714, metal pad 716, and a first bonding layer 712.The carrier wafer 730 includes a second bonding layer 732. The donorwafer 710 and the carrier wafer 730 are bonded to each other through thefirst bonding layer 712 and the second bonding layer 732. The substrate710 may be similar to substrate 502 of FIG. 5A. The absorption region706 may be similar to the absorption region 506 of FIGS. 5A-5L.

The pixels 720 a through 730 c include absorption regions 706 a through706 c, respectively, and microlenses 722 a through 722 c (collectedreferred to as microlenses 722), respectively. The microlenses 722 areconvex lenses that are integrated into or on the donor wafer 710. Inapplications that benefit from high light collection efficiency, such asToF detection, addition of microlenses 722 may be beneficial. The convexconfiguration of the microlens 722 can cause light incident on themicrolens 722 to be focused toward the absorption region 706, which mayimprove light collection efficiency of the pixels 720, leading toimproved pixel performance. The arrangement of the pixel 720 with themicrolens 722 on a backside of the donor wafer 710 may be referred to asbackside illumination.

The microlens 722 has various characteristics that affect itsperformance, including geometrical parameters and material from which itis formed. The microlens 722 is typically implemented in a plano-convexconfiguration, with one surface facing the incident light and beingconvex with a radius of curvature, and the other surface being a planarsurface interfacing with the donor wafer 710 in or on which themicrolens 722 is formed. The plano-convex configuration of the microlens722 may lend itself to fabrication through standard semiconductorprocessing techniques. The microlens 722 may have a height HL and adiameter DL, and may be separated from a lens-facing surface of theabsorption region 706 by a height Ho. In some implementations, HL mayrange from 1 to 4 μm, Ho may range from 8 to 12 μm, HA may range from 1to 1.5 μm, and DL may range from 5 to 15 μm. In some implementations,for a spherical-type microlens 722, its radius of curvature may be setsuch that the focal length of the microlens 722 is approximately equalto Ho to achieve optimal focusing of light onto the absorption region706. The determination of the focal length and the radius of curvaturemay be performed using various simulation techniques such as beampropagation method (BPM) and finite difference time domain (FDTD)technique. In some implementations, the microlens 722 is an asphericlens.

The microlens 722 can be formed from various materials and fabricated invarious ways. In general, various materials that are transparent for thewavelengths to be detected by the pixels 720 may be used. For example,the microlens 722 may be fabricated from materials having moderate tohigh index of refraction (e.g., >1.5), such as crystalline silicon,polysilicon, amorphous silicon, silicon nitride, polymer, or combinationthereof. For visible wavelengths, polymer materials may be used. For NIRwavelengths, silicon may be used as silicon is relatively transparent inthe NIR, and has a relatively high index of refraction (approximately3.5 at 1000 nm), making it well suited as a lens material in the NIR.Furthermore, as silicon is strongly absorbing in the visible wavelengths(e.g., <800 nm), a silicon microlens may block a substantial portion ofvisible light from reaching the absorption region 706, which may bebeneficial for applications where selective detection of NIR wavelengthsis desired (e.g., ToF detection). A crystalline silicon microlens 722may be fabricated by patterning and etching a surface of the donor wafer710, which is typically a crystalline silicon wafer. As another example,polysilicon or amorphous silicon may be deposited on the surface of thedonor wafer 710, which may then be patterned and etched in similarfashion. The formation of microlens 722 through etching of thecrystalline silicon donor wafer 710 or by etching of the polysilicon oramorphous silicon deposited on the donor wafer 710 is an example methodof integrally forming the microlens 722 on the donor wafer 710.

The patterning of the microlens 722 may be performed using, for example,grayscale lithography techniques. In grayscale lithography, a feature tobe patterned, such as the microlens, is exposed using a local gradationin the exposure dose, which translates into a gradation in the thicknessof the resulting photoresist mask that has been developed. For example,the photoresist mask can be patterned to have a similar shape as themicrolens 722. The photoresist mask is then transferred onto thematerial underneath, such as the crystalline silicon donor wafer 710, bysemiconductor etching techniques such as plasma-based directionaletching techniques, completing the fabrication of the microlens 722. Insome implementations, the local gradation in the exposure dose may beachieved, for example, by varying a fill-factor of sub-wavelengthfeatures on a photomask

The absorption regions 706 may be similar to absorption region 506described in relation to FIG. 5A. The carrier wafer 730 may includevarious electronic circuits that are coupled to the pixels 720. Forexample, the electronic circuits may be coupled through structures suchas the via 714. The via 714 may be coupled to a metal pad 716 tointerface with external electronics through, for example, a wire bond.

The carrier wafer 730 and the donor wafer 710 may be bonded ormechanically attached to one another through various techniques. Forexample, the first and second bonding layers 712 and 732 may be oxides(e.g., silicon dioxide), and the bonding may be an oxide-to-oxidebonding. As another example, the first and second bonding layers 712 and732 may be metals (e.g., copper), and the bonding may be ametal-to-metal bonding. As yet another example, the first and secondbonding layers 712 and 732 may be a combination of oxide and metals(e.g., silicon dioxide and copper), and the bonding may be a hybridbonding.

FIG. 7B shows a cross-sectional view of an example configuration 740 ofa microlens integrated with a photodetector. The configuration 740includes a microlens 742, an anti-reflection coating (ARC) layer 744, aspacer layer 746, a first layer 748, a second layer 750, a silicon layer752 and a photodetector 754. The ARC layer 744 is supported by themicrolens 742. The microlens 742 is supported by the spacer layer 746.The photodetector 754 may be supported by the silicon layer 752 or beformed within the silicon layer 752. The first layer 748 and the secondlayer 750 may be intermediate layers between the silicon layer 752 andthe spacer layer 746.

The ARC layer 744 is provided to reduce a reflection of light incidenton the microlens 742. The ARC layer 744, for example, may be designed tohave a refractive index that is the square root of the index of themicrolens 742, and have a thickness corresponding to a quarter of theincident wavelength. In some implementation, the ARC layer 744 may beformed from silicon dioxide. In some implementations, the ARC layer 744may include multiple layers to form a multi-layer ARC.

The configuration 740 may correspond to an integration of microlens 742in a back-side illuminated (BSI) image sensor configuration. Forexample, the silicon layer 752 can be a silicon substrate, such as thesubstrate 710 of FIG. 7A or substrate 502 of FIG. 5D, and thephotodetector 754 may be, for example, the switched photodetector 530 ofFIG. 5D. The interface between the silicon layer 752 and the secondlayer 750 may correspond to the bottom surface of the substrate 502opposite to the absorption region 506 of FIG. 5D. In such a BSIconfiguration, the second layer 750 formed on the silicon layer 752,e.g., the backside of the substrate 502, can include various structuresand layers typical in fabrication of a BSI illuminated sensor wafer.Examples of such structures and layers include an ARC layer for reducinglight reflection at the interface of the silicon layer 752, and a metalgrid, such as a tungsten grid, for blocking light into the silicon layer752 other than regions for receiving light, such as the regionsunderneath the microlens 742. The first layer 748 may be a thin layer ofmaterial that promotes adhesion of the spacer layer 746 to the secondlayer 750 for improving, among others, manufacturability and reliabilityof the configuration 740. The material for the first layer 748 may be,for example, various dielectric materials (e.g., SiO₂, SiON, and SiN) orpolymers. In some implementation, the first layer 748 can be omitteddepending on the interaction between the second layer 750 and the spacerlayer 746 (e.g., in the case where the spacer layer 746 has goodadhesion with the second layer 750).

The configuration 740 may be fabricated by providing a sensor waferincluding the silicon layer 752, the photodetector 754, and the secondlayer 750, and depositing the first layer 748, the spacer layer 746, themicrolens 742, and the ARC layer 744 in the order given, and thenpatterning and etching to expose metal pads similar to the metal pad 716shown in FIG. 7A. The microlens 742 may be patterned and etched usingtechniques described in relation to fabrication of the microlens 722 ofFIG. 7A. While the ARC layer 744 is shown to be limited to the surfaceof the microlens 742, in general, the ARC layer 744 may extend to othersurfaces, such as the side surface of the microlens 742 and the uppersurface of the spacer layer 746.

Various characteristics of the components of a particular implementationof the configuration 740 configured for operational wavelength of 940 nmare given as an example. The microlens 742 has a refractive index of1.5316, a radius of curvature of 6 μm, a height of 4 μm, and a diameterDL of 10 μm. The ARC layer 744 is formed from SiO₂, which has arefractive index of 1.46 at 940 nm and a thickness of 160.96 nm. Thespacer layer 746 has a refractive index of 1.5604, and a thickness of 10μm. The first layer 748 has a refractive index 1.5507 and a thickness of60 nm. The second layer 750 includes an ARC layer for the silicon layer752 and a tungsten grid. While specific characteristics have beenprovided, the characteristics may be modified to adapt the configuration740, for example, for different operational wavelengths, materials, andsize of the photodetector 754.

In some implementations, the second layer 750, which may be referred toas the “top layer” formed on top of the backside of a silicon substrateof a BSI image sensor, may be modified to improve the overall opticalperformance of configuration 740. The second layer 750, as previouslydescribed, typically includes metal grid embedded in a dielectric layer,such as tungsten grid embedded in a layer of SiO₂. This layer of SiO₂may serve as an ARC layer if the light was entering the silicon layer752 directly from air. However, due to the addition of the microlens742, the spacer layer 746 and the first layer 748 which all haverefractive indices that are significantly higher than that of air(approximately 1.0), the SiO₂ layer may not function effectively inreducing the optical reflection at the interface between the siliconlayer 752 and the stacking of the first layer 748 and/or spacer layer746.

Table 1 shows simulation parameters and calculated transmission of animplementation of configuration 740. The layers and the thicknesses havebeen adapted and/or approximated for the purpose of performing asimulation that approximate the expected transmission of differentimplementations of the configuration 740.

TABLE 1 REFRACTIVE THICKNESS (μm) LAYERS INDEX Case 1 Case 2 ARC layer744 1.249 0.188 Spacer layer 746 1.560 10 First layer 748 1.551 0.06Second SiO₂ 1.451 0.8 layer Si₃N₄ 1.949 0 0.120 750 Silicon layer 7523.599 + 0.00135i 1 Transmission (%) 78.95 97.62

Referring to Table 1, case 1 corresponds to a second layer 750 thatincludes a standard single layer of SiO₂, which results in a simulatedtransmission of approximately 79%. For applications where it isimportant to detect as much of the incident light as possible, such 21%loss of the incident light may not be acceptable. Such a drop intransmission can be mitigated by including a Si₃N₄ layer in the secondlayer 750 under the SiO₂ layer as an intermediate layer between the SiO₂layer and the silicon layer 752. By including approximately 121 nm ofSi₃N₄, the transmission can be improved to approximately 97.6%. As such,the intermediate layer may be referred to as an anti-reflection layer.In general, various optically transparent material with a refractiveindex greater than SiO₂ may be used in place of Si₃N₄. Example materialsinclude SiON, SiN, Al₂O₃, HfO₂, ZrO₂, and La₂O₃, and high-k materials(e.g., materials with high dielectric constant) that are compatible withCMOS manufacturing processes. Suitable material may have a refractiveindex greater than, for example, 1.6, 1.7, 1.8, 1.9, or 2.0. Thicknessof the material should be adapted to be an odd multiple of a quarter ofthe wavelength of light within the material.

The addition of Si₃N₄ or high-k material layer directly on top of thesilicon layer 752 may result in an increase of a dark current of thephotodetector 754 due to, for example, increased surface defect at theSilicon-Si₃N₄ interface relative to Silicon-SiO₂ interface. To mitigatesuch increase in dark current, in some implementations, a second layerof SiO₂ can be inserted between the Si₃N₄ layer and the silicon layer752. Inserting the second layer of SiO₂ of thickness ranging from 10 nmto 50 nm results in a transmission ranging from approximately 97.1% to85%, respectively. As such, inserting a thin layer of SiO₂, such as 10nm, may be beneficial for mitigating the increase in dark current whilemaintaining high optical transmission.

Low leakage current flowing across control regions of a switchedphotodetector, as previously described, is an important performanceparameter, as it contributes to lowering power consumption ofapparatuses including the photodetector. Another important aspectperformance parameter is dark current flowing between a readout regionand the control region of a switched photodetector, as the dark currentcontributes to the noise of a signal detected by the switchedphotodetector, degrading the signal-to-noise ratio (SNR) of a measureToF signal.

FIG. 8A shows an example switch 800 for a switched photodetector. Theswitch 800 may be used as a first or second switch in various switchedphotodetectors described in the present specification. The switch 800 isformed in the absorption region 506 having the first layer 508, whichhave been described previously described in relation to FIG. 5A. Theswitch 800 includes an n-doped region 802, a readout contact 804 coupledto the n-doped region 802, a lightly doped n-well region 806, a p-dopedregion 812, a control contact 814 coupled to the p-doped region 812, alightly doped p-well region 816, and an n-well region 818. The edges ofthe n-doped region 802 and the p-doped region 812 are separated by adistance S. The n-doped region 802 and the p-doped region 812 may besimilar to the first n-doped region 534 and the first p-doped region 537of FIG. 5E. The n-well region 818 may be similar to the n-well region539 in FIG. 5E. The readout contact 804 and the control contact 814 maybe similar to the first readout contact 535 and the first controlcontact 538 in FIG. 5E. The p-doped region 812 may be a control region,and the n-doped regions 802 may be a readout region.

Origins of the dark current in a lateral PIN diode formed by the controlregion (p-doped region 812), the absorption region 506(undoped/intrinsic), and a readout region (n-doped region 802) includeShockley-Read-Hall (SRH) generation and band-to-band tunneling. SRHgeneration may be influenced by presence of surface defects at thesurface of the absorption region 506. The addition of the first layer508 partially reduces the surface defect, which can reduce the darkcurrent due to SRH generation. Increasing the distance S between then-doped region 802 and the p-doped region 812 can also reduce the darkcurrent due to, for example, lowering of the electrical field betweenthe n-doped region 802 and the p-doped region 812, which in turndecreases the SRH generation rate between the said regions. For example,the distance S should be kept at above 400 nm. However, increasing thedistance S can lead to a reduction in bandwidth of the photodetector dueto, for example, an increase in carrier transit time. Addition of thelightly doped n-well region 806, the lightly doped p-well region 816, orcombinations thereof may help overcome such tradeoff.

The respective lightly doped regions 806 and 816 have dopantconcentrations that are lower than the respective n-doped region 802 andthe p-doped region 812. For example, the lightly doped regions 806 and816 can have dopant concentrations on the order of 1*10¹⁷ cm⁻³, whichare lower than that of the n-doped region 802 and the p-doped region 812which can have dopant concentrations on the order of 1*10¹⁹ cm⁻³. Thepresence of the lightly doped regions reduces discontinuity in thedopant concentrations between the doped regions 802 and 812 and theabsorption region 506, which may have dopant concentrations on the orderof 1*10¹⁵ cm⁻³ or lower, by providing a region of intermediate dopantconcentration, which results in a reduction in the electric field valuesat the edges of the doped regions 802 and 812. By reducing the electricfield values, band-to-band tunneling may also be reduced, which leads tolowering of the dark current between the two doped regions 802 and 812.In addition, contributions from SRH generation may be reduced. Ingeneral, the doping concentration of the lightly doped regions 806 and816 may be set based on various factors such a geometry of the switch,doping concentration of the doped regions 802 and 812, and dopingconcentration of the absorption region 506.

FIG. 8B shows an example switch 820 for a switched photodetector. Switch820 is similar to the switch 800 in FIG. 8A, but differs in that insteadof the lightly doped regions 806 and 816, a trench 822 is formed in theabsorption region 506, which is filled by a dielectric fill 824. Thetrench 822 filled with the dielectric fill 824 can contribute to areduction in the dark current.

The dielectric fill 824 is typically an electrically insulating materialwith a dielectric constant lower than that of the surrounding absorptionregion 506. Electric field is able to penetrate further into a region oflow dielectric constant compared to region of high dielectric constant.By placing the dielectric-filled trench 822 in proximity to the dopedregions 802 and 812, some of the high electric field regions formedaround the doped regions 802 and 812 and in depletion regions (“spacecharge region”) surrounding the doped regions 802 and 812 are pulledinto the dielectric fill 824. Accordingly, SRH generation and/orband-to-band tunneling in the absorption region 506 is reduced.Furthermore, unlike the germanium absorption region 506, the dielectricfill 824 such as SiO₂ is an insulator and does not contribute to SRHgeneration and/or band-to-band tunneling. Therefore, dark currentgeneration through SRH generation and/or band-to-band tunneling that iscaused by high electric field regions at the edges of the doped regions802 and 812 may be reduced.

The trench 822 may be formed by etching the absorption region throughdry (e.g., plasma etching) or wet (e.g., liquid chemical bath) etchingtechniques. The trench 822 may be etched to a depth similar to the depthof the doped regions 802 and 812 (e.g., 10-200 nm). The trench 822should overlap with at least a portion of high electric field regionssurrounding at least one of the n-doped region 802 or the p-doped region812. In some implementations, the trench 822 cuts into the doped regions802 and 812, removing a portion of the n-doped region 802 and thep-doped region 812. Once the trench 822 is formed, the first layer 508may be deposited over the trench 822 to passivate the defects present onthe surface of the trench 822. In the case of a germanium absorptionregion 806, the first layer 508 may be, for example, amorphous silicon,polysilicon, germanium-silicon, or a combination thereof. Then, thetrench 822 is filled with the dielectric fill 824, which may be, forexample, SiO₂. The dielectric fill 824 should be clean withoutsignificant concentration of impurities to avoid generation of darkcurrent.

In some implementations, the depth of the trench may be deeper than thedepth of the doped regions 802 and 812. For example, for doped regions802 and 812 that are approximately 100 nm deep, a trench depth of 200 nmmay further reduce SRH generation and/or band-to-band tunneling. In someimplementations, greater than 50% reduction in SRH generation and/orband-to-band tunneling around the doped regions 802 and 812 may beobserved.

FIG. 8C shows an example switch 830 for a switched photodetector. Theswitch 830 is similar to the switch 800 of FIG. 8A, but further includesthe trench 822 and dielectric fill 824 of FIG. 8B. By simultaneouslyimplementing the lightly-doped regions 806 and 816 and the trench 822,the band-to-band tunneling, the SRH recombination, or combinationthereof may be further reduced over individually implementing either thelightly-doped regions 806 and 816 or the trench 822 in isolation.

In general, the reduction in dark current through the use of lightlydoped regions 806 and 816 or the trenches 822 depends on the specificdesign of the switch and the overall design of the switchedphotodetector that includes the switch. As such, while theimplementation shown in FIG. 8C includes both the lightly doped regions806 and 816 and the trenches 822, a decision to implement the lightlydoped regions, the trench, or combination of the two may be based on thespecific design of the switched photodetector in which the switch is tobe included. Furthermore, while a single trench 822 is shown, ingeneral, the trench 822 may be split into two or more trenches.

While the first layer 508 and the n-well 818 is included in theimplementations shown in FIGS. 8A-8D, the first layer 508, the n-well818, or both may be omitted in some implementations.

So far, various implementations of switched photodetectors and switchesfor the switched photodetectors have been described. Now, details of thevarious structures and components of switched photodetectors will bedescribed.

A switched photodetector is typically fabricated on a substrate, such assubstrate 102, 202, 302, 402, and 502. The substrate is a carriermaterial on which the switched photodetector is fabricated. Asemiconductor wafer is an example of a substrate. The substrate may bepart of the switched photodetector, but in general, the substrate maysimply provide a mechanical platform on which the switched photodetectoris fabricated. The substrate may be formed from different materials,such as Silicon, Germanium, compound semiconductors (e.g., III-V,II-VI), Silicon Carbide, glass, and sapphire. The substrate may includevarious layers within. For example, a Silicon-on-Insulator (SOI)substrate includes a base layer of silicon, an insulator layer (e.g.,SiO₂) on the base layer of silicon, and a device layer of silicon on thelayer of insulator. The SOI may include additional devicelayer-insulator layer pairs. For example, a dual-SOI (DSOI) waferincludes two device layer-insulator layer pairs.

A switched photodetector includes an absorption region configured toabsorb incident light and convert the absorbed light into chargecarriers. Absorption layers 106, 206, 306, and 406, and absorptionregions 506 and 706 are examples of the absorption region. Theabsorption region may be formed from various absorber materials thatabsorb the light at the operational wavelengths of the switchedphotodetector. Example materials for the absorption region includeSilicon, Germanium, IV-IV semiconductor alloy (e.g., GeSn, GeSi), III-Vcompound semiconductors (e.g., GaAs, InGaAs, InP, InAlAs, InGaAlAs), andother materials in the group III, IV, and V of the periodic table. Insome implementations, absorption region may be a region within thesubstrate. For example, a region of a silicon substrate may be used asan absorption region for visible light.

In some implementations, the absorption region may be defined within alight-absorbing material by a change in material composition (e.g.,different GeSi composition), by doping a region within the absorbingmaterial (e.g., counter doped region), or by forming an optical windowto pass through light (e.g., tungsten grid openings in a BSI imagesensor).

The absorber material may be deposited on the substrate. For example,absorber material may be blanket-deposited on the substrate. In someimplementations, the absorber material may be deposited on anintermediate layer formed on the substrate. In general, the intermediatelayer may be selected based on the absorber material, the substrate, orboth. Such intermediate layer may improve device manufacturabilityand/or improve device performance. Example materials for theintermediate layer include silicon, graded germanium-silicon compoundmaterial, graded III-V material, germanium, GaN, and SiC. Gradedmaterial refers to a material that has a varying material compositionalong at least one direction. For example, a graded GeSi material mayhave a composition that varies from 1% Germanium on one end of thematerial to 99% Germanium of the other end of the material. In general,the starting and ending composition may be set, for example, based onthe substrate composition and the absorber material composition.

In some implementations, the absorber material can be epitaxially grownon the intermediate layer in two or more steps. For example, theabsorber material (e.g., Ge, GeSi) may be deposited on a dielectriclayer with openings to underlying substrate (e.g., crystalline Siliconsubstrate). Such multi-step growth procedure may improve materialquality (e.g., reduced number of material defects) when the absorbermaterial is deposited on a substrate having mismatched latticeconstants. Examples of such multi-step growth procedure is described inU.S. Pat. No. 9,786,715 titled “High Efficiency Wide Spectrum Sensor,”which is fully incorporated by reference herein.

FIGS. 9A-9D show example electrical terminals for use in switchedphotodetectors. Referring to FIG. 9A, an electrical terminal 900 includea region 902, and a contact metal 904, and a doped region 906. Theregion 902 is a material on which the electrical terminal 900 is formed,and may correspond to an absorption region, such as the absorptionregion 506, or a substrate, such as the substrate 502. The doped region906 may be a p-type (acceptor) doped region or an n-type (donor) dopedregion depending on the type of dopant. The doped region 906 istypically doped to a high doping concentration (e.g., between 1*10¹⁹ to5*10²⁰ cm⁻³) to allow an Ohmic contact to be formed between the contactmetal 904 and the region 902. Such level of doping concentration may bereferred to as “degenerate doping.”

The contact metal 904 is a metallic material that is in contact with theregion 902 through the doped region 906. The contact metal may beselected from various metals and alloys based on the material of theregion 902 and dopants of the doped region 906. Examples include Al, Cu,W, Ti, Ta—TaN—Cu stack, Ti—TiN—W stack, and various silicides.

Referring to FIG. 9B, an electrical terminal 910 is similar to theelectrical terminal 900 of FIG. 9A, but differs in that the doped region906 is omitted. The direct placement of the contact metal 904 on theregion 902 without the doped region 906 may lead to formation of aSchottky contact, an Ohmic contact, or a combination thereof having anintermediate characteristic between the two, depending on variousfactors including the material of the region 902, the contact metal 904,and the impurity or defect level of the region 902.

Referring to FIG. 9C, an electrical terminal 920 is similar to theelectrical terminal 910 of FIG. 9B, but differs in that a dielectriclayer 922 is inserted between the contact metal 904 and the region 902.For example, for a (crystalline) germanium region 902, the dielectriclayer 922 may be amorphous silicon, polysilicon, or germanium-silicon.As another example, for a (crystalline) silicon region 902, thedielectric layer 922 may be amorphous silicon, polysilicon, orgermanium-silicon. The insertion of the dielectric layer 922 may lead toformation of a Schottky contact, an Ohmic contact, or a combinationthereof having an intermediate characteristic between the two.

Referring to FIG. 9D, an electrical terminal 930 is similar to theelectrical terminal 910 of FIG. 9B, but differs in that an insulatinglayer 932 is inserted between the contact metal 904 and the region 902.The insulating layer 932 prevents direct current conduction from thecontact metal 904 to the region 902, but allows an electric field to beestablished within the region 902 in response to an application of avoltage to the contact metal 904. The established electric field mayattract or repel charge carriers within the region 902. The insulatinglayer 932 may be SiO₂, Si₃N₄, or high-k material.

A switch, such as the switch first switch 532 of FIG. 5D, of a switchedphotodetector includes a carrier control terminal and a carriercollection (readout) terminal. A carrier control terminal is a terminalconfigured to direct photo-generated carriers within the region 902 in acertain direction (e.g., toward the carrier collection terminal) byapplication of a control voltage through, for example, an external biascircuitry. The operation of the carrier control terminal has beendescribed in relation to the control signals 122 and 132 of FIG. 1A.Different types of electrical terminals may be used to implement thecarrier control terminal. For example, the electrical terminals 900,910, 920, and 930 may be used to implement the carrier control terminal.

A carrier collection terminal is a terminal configured to collect thephoto-generated carriers in the region 902. The carrier collectionterminal may be configured to collect electrons (e.g., n-type dopedregion 906) or holes (e.g., p-type doped region 906). The operation ofthe carrier collection terminal has been described in relation to thereadout circuits 124 and 134 of FIG. 1A. Different types of electricalterminals may be used to implement the carrier collection terminal. Forexample, the electrical terminals 900, 910, and 920 may be used toimplement the carrier collection terminal.

The number of carrier control and carrier collection terminals may bevaried based on a variety of considerations, such as target deviceperformance. As examples, a switched photodetector may have thefollowing exemplary configurations: 2 carrier control terminals and 2carrier collection terminals; 2 carrier control terminals and 1 carriercollection terminal; 4 carrier control terminals and 2 carriercollection terminals; and 4 carrier control terminals and 4 carriercollection terminals. In general, a switched photodetector can have anynumber of carrier control terminals and carrier collection terminalsgreater than 1.

When more two or more control terminals are implemented within aswitched photodetector, various combinations of the previously describedelectrical terminals may be used. For example, a combination of Ohmicand Schottky/Ohmic terminals (e.g., terminals 900 and 920), Ohmic andinsulating (e.g., terminals 900 and 930), insulating and Schottky/Ohmic(e.g., terminals 930 and 920), and Ohmic and Schottky/Ohmic, andinsulating (e.g., terminals 900, 920, and 930) may be used.

When more two or more carrier collection terminals are implementedwithin a switched photodetector, a combination of Ohmic andSchottky/Ohmic terminals (e.g., terminals 900 and 920) may be used.

The electrical terminals can have various shapes based on a variety ofconsiderations, such as manufacturability and device performance. FIG.9E shows an example top view of various shapes of an electricalterminal. The terminals 940 may have rectangular, triangular, circular,polygonal, or may be a combination of such shapes. The corners of theterminals may be sharp, or may be rounded. The shapes can be definedusing doping region, metal silicide, contact metal or any combinationthereof.

The absorption region and the substrate may be arranged in variousconfigurations, and the absorption region may take on various shapesbased on various considerations, such as manufacturability and deviceperformance. Referring to FIGS. 10A-10I, example configurations of anabsorption region and a substrate are shown. Specifically, referring toFIG. 10A, a configuration 1000 includes a substrate 1002 and anabsorption region 1004 protruding from an upper surface of the substrate1002. The substrate 1002 may be similar to substrate 502 described inrelation to FIG. 5D, and the absorption region 1004 may be similar toabsorption region 506 described in relation to FIG. 5D. Theconfiguration 1000 may be fabricated by depositing the absorption region1004 on the substrate 1002, and etching the absorption region 1004 intothe protruding structure.

Referring to FIG. 10B, a configuration 1010 is similar to theconfiguration 1000 in FIG. 10A, but now includes an intermediate layer1006 between the absorption region 1004 and the substrate 1002. Theintermediate layer may be a buffer layer that facilitates the growth ofthe absorption region 1004 over the substrate 1002. The configuration1010 may be fabricated by depositing an intermediate layer 1006 on thesubstrate 1002, depositing the absorption region 1004 on theintermediate layer 1006, and etching the absorption region 1004 and theintermediate layer 1006 into the protruding structure.

Referring to FIG. 10C, a configuration 1020 is similar to theconfiguration 1000 in FIG. 10A, but now the absorption region 1004 ispartially embedded in the substrate 1002. The configuration 1020 may befabricated by forming a recess on the substrate 1002, and selectivelydepositing the absorption region 1004 in the formed recess.Alternatively, the configuration 1020 may be fabricated by depositing asacrificial layer over the substrate 1002, etching through the depositedsacrificial layer to form the recess on the substrate 1002, selectivelydepositing the absorbing material, removing the absorbing materialdeposited outside of the recess by performing a planarizing step such asa chemical-mechanical polishing (CMP) step, and removing the sacrificiallayer through a selective etch, such as a wet chemical etch.

Referring to FIG. 10D, a configuration 1030 is similar to theconfiguration 1020 in FIG. 10C, but now the absorption region 1004 isfully embedded in the substrate 1002. The configuration 1030 may befabricated by forming a recess on the substrate 1002, depositing aselective layer of absorbing material over the substrate 1002, andremoving the absorbing material deposited outside of the recess byperforming a planarizing step, such as a chemical-mechanical polishing(CMP) step.

Referring to FIG. 10E, a configuration 1040 is similar to theconfiguration 1030 in FIG. 10D, but now the intermediate layer 1006 isinserted between, in the recess, the absorption region 1004 and thesubstrate 1002. The configuration 1040 may be fabricated by forming arecess on the substrate 1002, depositing a conformal layer of theintermediate layer 1006, depositing a blanket layer of absorbingmaterial over the intermediate layer 1006, and removing the absorbingmaterial and the intermediate layer deposited outside of the recess byperforming a planarizing step, such as a chemical-mechanical polishing(CMP) step.

Referring to FIG. 10F, a configuration 1050 is similar to theconfiguration 1040 in FIG. 10E, but now a second intermediate layer 1008replaces the first intermediate layer 1006 at the interface between asidewall of the absorption region 1004 and the sidewall of the recess ofthe substrate 1002. The configuration 1050 may be fabricated by forminga recess on the substrate 1002, depositing a conformal layer of thesecond intermediate layer 1008, performing an anisotropic blanketetching to remove the second intermediate layer 1008 along verticalsurfaces, depositing a conformal layer of the first intermediate layer1006, performing an anisotropic blanket etching to remove the firstintermediate layer 1006 along non-vertical surfaces, depositing aselective layer of absorbing material, and removing the absorbingmaterial and the first intermediate layer deposited outside of therecess by performing a planarizing step, such as a chemical-mechanicalpolishing (CMP) step. In an exemplary implementation, the firstintermediate layer 1006 may be formed from SiO₂, and the secondintermediate layer 1008 may be formed from GeSi.

Referring to FIG. 10G, a configuration 1060 is similar to theconfiguration 1000 in FIG. 10A, but now includes a tiered intermediatelayer 1062 in which the absorption region 1004 is embedded. The tieredintermediate layer 1062 includes an opening 1064 to the substrate 1002,and a recess 1066 in which the absorption region 1004 is embedded. Theabsorption region 1004 contacts the substrate 1002 through the opening1064. The configuration 1060 may be fabricated by depositing anintermediate layer on the substrate 1002, etching the opening 1064throughout the entire thickness of the deposited intermediate layer,etching the recess 1066 in the deposited intermediate layer, depositingthe absorption region 1004 on the tiered intermediate layer 1062, andremoving the absorbing material deposited outside of the recess 1066 byperforming a planarizing step, such as a chemical-mechanical polishing(CMP) step.

Referring to FIG. 10H, a configuration 1070 is similar to theconfiguration 1060 in FIG. 10G, but now includes a second intermediatelayer 1072 in which the recess 1066 is formed. The configuration 1070may be fabricated by depositing the first intermediate layer 1062 on thesubstrate 1002, depositing the second intermediate layer 1072, etchingthe opening 1064 through the first intermediate layer 1062 and thesecond intermediate layer 1072, etching the recess 1066 in the secondintermediate layer 1072, depositing the absorption region 1004, andremoving the absorbing material deposited outside of the recess 1066 byperforming a planarizing step, such as a chemical-mechanical polishing(CMP) step.

Referring to FIG. 10I, a configuration 1080 is similar to theconfiguration 1040 in FIG. 10E, but now includes an opening 1084 formedon the intermediate layer 1006. The absorption region 1004 contacts thesubstrate 1002 through the opening 1084. The configuration 1080 may befabricated by forming a recess on the substrate 1002, depositing aconformal layer of the intermediate layer 1006, etching the opening1084, depositing a blanket layer of absorbing material over theintermediate layer 1006, and removing the absorbing material and theintermediate layer deposited outside of the recess by performing aplanarizing step, such as a chemical-mechanical polishing (CMP) step.

The absorption region, the carrier control terminals, and the carriercollection terminals may be arranged in various configurations based ona variety of considerations, such as manufacturability and deviceperformance. FIGS. 11A-11B show a top view and a side view of an exampleswitched photodetector 1100 in which the carrier control terminals andthe carrier collection terminals are placed on the substrate, and aportion of the substrate is the absorption region. In this example, theswitched photodetector 1100 includes a substrate 1102, an absorptionregion 1104, carrier collection terminals 1106, and carrier controlterminals 1108. The absorption region 1104 is a region within thesubstrate 1102. For example, for a silicon substrate 1102, theabsorption region 1104 is formed in silicon, and the absorption region1104 may absorb visible light. The absorption region 1104 may havevarious shapes, e.g., a square shape in a top view of the photodetector1100. The absorption region 1104 may extend from an upper surface of thesubstrate 1102 and into a desired depth below the upper surface. Forexample, the absorption region 1104 may extend 1 μm, 2 μm, 3 μm, 5 μm,or 10 μm below the upper surface of the substrate 1102. Adjacent pairsof the carrier collection terminal 1106 and the carrier control terminal1108 forms a switch. The absorption region 1104 is arranged between theadjacent pairs of carrier collection terminal 1106 and the carriercontrol terminal 1108. In some implementations, the adjacent pairs ofcarrier collection terminal and carrier control terminal are arrangedsymmetrically about the absorption region 1104 (e.g., on opposite sidesor on the four sides of the absorption region 1104). Such symmetricplacement may improve matching of carrier control and collectionperformance of the two switches formed by the pairs.

FIGS. 11C-11F show a top view and side views of example switchedphotodetectors in which the absorption regions are formed from amaterial different than the substrate. Referring to FIGS. 11C-11D, theswitched photodetector 1120 includes the substrate 1102, an absorptionregion 1124, the carrier collection terminals 1106, and the carriercontrol terminals 1108. FIG. 11C shows a top view of the switchedphotodetector 1120, and FIG. 11D shows a side view of the switchedphotodetector 1120. The switched photodetector 1120 is similar to theswitched photodetector 1100 of FIGS. 11A-11B, but differs in that theabsorption region 1124 of the switched photodetector 1120 is formed froma material different than the substrate 1102. For example, theabsorption region 1124 may be formed from germanium, and the substrate1102 may be a silicon substrate. The absorption region 1124 is fullyembedded in a recess formed in the substrate 1102. While specifics ofthe embedded structure are not shown, the embedded absorption region1124 may be implemented, for example, as described in relation to FIGS.10D-10F and FIG. 5C.

Referring to FIG. 11E, a switched photodetector 1130 is similar to theswitched photodetector 1120 of FIGS. 11C-11D, but differs in that theabsorption region 1124 is now partially embedded in the substrate 1102.While specifics of the partially embedded structure are not shown, thepartially embedded absorption region 1124 may be implemented, forexample, as described in relation to FIG. 10C and FIG. 5B.

Referring to FIG. 11F, a switched photodetector 1140 is similar to theswitched photodetector 1120 of FIGS. 11C-11D, but differs in that theabsorption region 1124 is now fully protruding on the substrate 1102.While specifics of the fully protruding structure are not shown, thefull protruding absorption region 1124 may be implemented, for example,as described in relation to FIGS. 10A-10B and FIG. 5A.

In some configurations of the switched photodetectors, the carriercollection terminals, the carrier control terminals, or both may beplaced on an absorption region. Descriptions of the implementationdetails of the substrate, the absorption region, the carrier controlterminals, and the carrier collection terminals will be omitted forbrevity. FIGS. 12A-12B show a top view and a side view of an exampleswitched photodetector 1200 in which the carrier collection terminalsare placed on the substrate, and the carrier control terminals areplaced on an absorption region. The switched photodetector 1200 includesa substrate 1202, an absorption region 1204, a light receiving region1205, carrier collection terminals 1206, and carrier control terminals1208. The light receiving region 1205 may indicate a portion of theabsorption region 1204 on which input light is incident, and may bephysically indistinguishable from the remaining portion of theabsorption region 1204. For example, a combination of a light shield(e.g., tungsten grid) and a microlens may block and focus the incidentlight onto the light receiving region 1205. The carrier collectionterminals 1206 are placed on the substrate 1202, and the carrier controlterminals 1208 are placed on the absorption region 1204 on a locationthat does not overlap with the light receiving region 1205. For theswitched photodetector 1200, the absorption region 1204 is fullyprotruding. The absorption region 1204 may be partially embedded asshown in FIG. 12C for a switched photodetector 1220, or fully embeddedas shown in FIG. 12D for a switched photodetector 1230.

FIGS. 12E-12F show a top view and a side view of an example switchedphotodetector 1240 in which both the carrier collection terminals andthe carrier control terminals are placed on the absorption region. Theswitched photodetector 1240 is similar to the switched photodetector1200 of FIGS. 12A-12B, but differs in that the carrier collectionterminals 1206 are now placed on the absorption region 1204 on alocation that does not overlap with the light receiving region 1205. Forthe switched photodetector 1240, the absorption region 1204 is fullyprotruding. The absorption region 1204 may be partially embedded asshown in FIG. 12G for a switched photodetector 1250, or fully embeddedas shown in FIG. 12H for a switched photodetector 1260.

While light receiving regions 1205 in FIGS. 12A-12H are shown to notoverlap with the carrier collection terminals or the carrier controlterminals, in general, the light receiving regions 1205 may overlap withat least a portion of the carrier control terminals, at least a portionof the carrier collection terminals, and at least a portion of thevarious n-doped regions or p-doped regions. For example, such overlapmay be present for a pixel that is used in both FSI and BSIconfigurations.

In some configurations of the switched photodetectors, each switch mayinclude more than one carrier collection terminals, more than onecarrier control terminals, or more than one of both. Descriptions of theimplementation details of the substrate, the absorption region, thelight receiving region, the carrier control terminals, and the carriercollection terminals will be omitted for brevity. FIGS. 13A-13G show topviews of example switched photodetectors having switches that includemultiple carrier control terminals or multiple carrier collectionterminals. Referring to FIG. 13A, the switched photodetector 1300includes a substrate 1302, an absorption region 1304, a light receivingregion 1305, substrate carrier collection terminals 1306, substratecarrier control terminals 1308, and absorber carrier control terminals1309. The substrate carrier collection terminals 1306 are carriercollection terminals placed on a substrate, such as the substrate 1302.The substrate carrier control terminals 1308 are carrier controlterminals placed on a substrate, such as the substrate 1302. Theabsorber carrier control terminals 1309 are carrier control terminalsplaced on an absorption region, such as the absorption region 1304. Theeffects and implementation details of the absorber carrier controlterminals 1309 in combination with the substrate carrier controlterminal 1308 have been described in relation to FIG. 5K. In someimplementations, the illustrated arrangement of the substrate carriercollection terminals 1306, the substrate carrier control terminals 1308,and the absorber carrier control terminals 1309 may be repeated in asecond row as shown in FIG. 13B.

Referring to FIG. 13B, the switched photodetector 1310 is similar to theswitched photodetector 1300 of FIG. 13A, but differs in that thesubstrate carrier control terminals 1308 has been omitted, and a secondrow of pairs of terminals 1306 and 1309 has been added. The second pairsof control and collection terminals may function independent of, orfunction in combination with the first pairs of control and collectionterminals that are adjacent to the second pairs of terminals.

Referring to FIG. 13C, the switched photodetector 1320 is similar to theswitched photodetector 1310 of FIG. 13B, but differs in that one of thesubstrate carrier collection terminals 1306 has been removed from eachside of the light receiving region 1305. The pairs of absorber carriercontrol terminals 1309 on each side of the light receiving region 1305in combination with respective substrate carrier collection terminals1306 may function as a switch.

Referring to FIG. 13D, the switched photodetector 1330 is similar to theswitched photodetector 1310 of FIG. 13B, but differs in that thesubstrate carrier collection terminals 1306 have been moved onto theabsorption region 1304 as absorber carrier collection terminals 1307.

Referring to FIG. 13E, the switched photodetector 1340 is similar to theswitched photodetector 1330 of FIG. 13D, but differs in that one of theabsorber carrier collection terminals 1307 has been removed from eachside of the light receiving region 1305. The pairs of absorber carriercontrol terminals 1309 on each side of the light receiving region 1305in combination with respective absorber carrier collection terminals1307 may function as a switch.

Referring to FIG. 13F, the switched photodetector 1350 is similar to theswitched photodetector 1330 of FIG. 13D, but differs in that one of theabsorber carrier control terminals 1309 has been removed from each sideof the light receiving region 1305. The pairs of absorber carriercollection terminals 1307 on each side of the light receiving region1305 in combination with respective absorber carrier control terminals1309 may function as a switch.

Referring to FIG. 13G, the switched photodetector 1360 is similar to theswitched photodetector 1330 of FIG. 13D, but differs in that four pairsof absorber carrier collection and control terminals 1307 and 1309 arenow symmetrically arranged about the light receiving region 1305. Eachpair of terminals 1307 and 1309 may function as a switch. Each switchmay function independently, or function in tandem with another switch.For example, east and west switches may be controlled as a first switchand north and south switches may be controlled as a second switch. Asanother example, the east and south switches may be controlled as afirst switch and west and north switches may be controlled as a secondswitch.

While light receiving regions 1305 in FIGS. 13A-13G are shown to notoverlap with the carrier collection terminals or the carrier controlterminals, in general, the light receiving regions 1305 may overlap withat least a portion of the carrier control terminals, at least a portionof the carrier collection terminals, and at least a portion of thevarious n-doped regions or p-doped regions. For example, such overlapmay be present for a pixel that is used in both FSI and BSIconfigurations.

For switches having two or more carrier control terminals, the carriercontrol terminals may be biased independently with independentlycontrolled bias voltages, or the carrier control terminals may beshorted together and biased with a single bias voltage. FIGS. 14A-14Bshow top views of example switched photodetectors having switches thatinclude multiple carrier control terminals. Referring to FIG. 14A, theswitched photodetector 1400 is similar to the switched photodetector1300 of FIG. 13A. The substrate carrier collection terminal 1306, thesubstrate carrier control terminal 1308, and the absorber carriercontrol terminal 1309 on the left side of the light receiving region1305 forms a first switch 1410. The substrate carrier collectionterminal 1306, the substrate carrier control terminal 1308, and theabsorber carrier control terminal 1309 on the right side of the lightreceiving region 1305 forms a second switch 1420.

Within the switches 1410 and 1420, the substrate carrier controlterminal 1308 and the absorber carrier control terminal 1309 may beshorted together and biased with a single bias voltage, or biased withindependently controlled bias voltages. For example, the substratecarrier control terminal 1308 of the first switch 1410 is biased withvoltage V_(B1) and the absorber carrier control terminal 1309 is biasedwith voltage V_(A1). Similarly, the substrate carrier control terminal1308 of the second switch 1420 is biased with voltage V_(B2) and theabsorber carrier control terminal 1309 is biased with voltage V_(A2). Insome implementations, the control terminals closer to the lightreceiving region 1305, such as the absorber carrier control terminals1309, may be biased to respective control voltages V_(A1) and V_(A2) todirect the photo-generated carriers in the light receiving region 1305toward the substrate carrier collection terminals 1306 that are biasedto voltages V_(c1 and) V_(c2) as shown. Simultaneously, the substratecontrol terminals 1308 may be biased to voltages V_(b1and) V_(b2) toestablish a high electric field between the substrate control terminals1308 and the substrate carrier collection terminals 1306. Withsufficiently high electric field between the terminals 1308 and 1306, aregion of avalanche multiplication may be established between theterminals 1308 and 1306, providing an avalanche gain to thephoto-generated carriers that have been directed toward the substratecarrier collection terminal 1306 by the absorber carrier controlterminal 1309. As a result, the photo-generated carrier may bemultiplied by an avalanche gain, which may increase the photocurrentsignal generated by the switched photodetector 1400.

Referring to FIG. 14B, the switched photodetector 1430 is similar to theswitched photodetector 1400 of FIG. 14A, but differs in that thesubstrate carrier collection terminals 1306 have been relocated onto theabsorption region 1304 as absorber carrier collection terminals 1407,and the substrate carrier control terminals 1308 have been relocatedonto the absorption region 1304 as absorber carrier control terminals1409. The effects of the different biases to the terminals are analogousto the effects described in relation to FIG. 14A.

While light receiving regions 1305 in FIGS. 14A-14B are shown to notoverlap with the carrier collection terminals or the carrier controlterminals, in general, the light receiving regions 1305 may overlap withat least a portion of the carrier control terminals, at least a portionof the carrier collection terminals, and at least a portion of thevarious n-doped regions or p-doped regions. For example, such overlapmay be present for a pixel that is used in both FSI and BSIconfigurations.

In typical implementations of an image sensor, multiple sensor pixels(e.g., photodetectors) are arranged in an array to allow the imagesensor to capture images having multiple image pixels. To allow highintegration density, multiple sensor pixels are typically arranged inclose proximity to each other on a common substrate. For asemiconducting substrate, such as p-doped silicon substrates, theproximity of the sensor pixels to each other may cause electrical and/oroptical crosstalk between the sensor pixels, which may, for example,decrease a signal to noise ratio of the sensor pixels. As such, variousisolation structures may be implemented to improve electrical isolationbetween the sensor pixels.

FIGS. 15A-15G show cross-sectional views of example configurations ofsensor pixel isolation. Referring to FIG. 15A, an example configuration1500 includes a substrate 1502, sensor pixels 1510 a and 1510 b(collectively referred to as sensor pixels 1510), and an isolationstructure 1506. The sensor pixels 1510 a and 1510 b include respectiveabsorption regions 1504 a and 1504 b. Each sensor pixels 1510 may be aswitched photodetector, such as the switched photodetectors of FIGS.5A-5L. Details of the sensor pixels 1510 has been omitted for clarity.

The isolation structure 1506 may increase the electrical isolationbetween the sensor pixels 1510 a and 1510 b. In configuration 1500, theisolation structure extends from an upper surface of the substrate 1502and extends into a predetermined depth from the upper surface. In someimplementations, the isolation structure 1506 is a doped region that hasbeen doped with p-type dopants or n-type dopants. The doping of theisolation structure 1506 may create a bandgap offset-induced potentialenergy barrier that impedes a flow of current across the isolationstructure 1506 and improving electrical isolation between the pixels1510 a and 1510 b. In some implementations, the isolation structure 1506is a trench filled with a semiconductor material that is different fromthe substrate 1502. An interface between two different semiconductorsformed between the substrate 1502 and the isolation structure 1506 maycreate a bandgap offset-induced energy barrier that impedes a flow ofcurrent across the isolation structure 1506 and improving electricalisolation between the pixels 1510 a and 1510 b.

In some implementations, the isolation structure 1506 is a trench filledwith a dielectric or an insulator. The isolation structure 1506 filledwith a low conductivity dielectric or insulator may provide a region ofhigh electrical resistance between the sensors pixels 1510 a and 1510 b,impeding a flow of current across the isolation structure 1506 andimproving electrical isolation between the pixels 1510 a and 1510 b.

While a single isolation structure 1506 has been shown, in general,there may be multiple isolation structures 1506 arranged between eachneighboring pairs of sensor pixels 1510. For example, in a 2D array ofsensor pixels 1510, a single sensor pixel 1510 may be surrounded by fournearest-neighbor sensor pixels 1510. In such a case, the isolationstructure 1506 may be placed along the four nearest-neighbor interfaces.In some implementations, the isolation structure 1506 may be acontinuous structure that surround the sensor pixel 1510. The isolationstructure 1506 may be shared at the interfaces between the pixels 1510.

Referring to FIG. 15B, an example configuration 1520 is similar to theconfiguration 1500 of FIG. 15A, but differs in that the absorptionregions 1504 a and 1504 b are fully embedded in the substrate 1502.

Referring to FIG. 15C, an example configuration 1530 is similar to theconfiguration 1500 of FIG. 15A, but differs in that the isolationstructure 1506 extends from the upper surface of the substrate 1502 tothe lower surface of the substrate 1502 through the entire depth of thesubstrate 1502. Configuration 1530 may remove alternative conductionpaths between the sensor pixels 1510 that diverts the isolationstructure 1506, and may improve electrical isolation between the sensorpixels 1510.

Referring to FIG. 15D, an example configuration 1540 is similar to theconfiguration 1530 of FIG. 15C, but differs in that the absorptionregions 1504 a and 1504 b are fully embedded in the substrate 1502.

Referring to FIG. 15E, an example configuration 1550 includes thesubstrate 1502, the sensor pixels 1510 a and 1510 b (collectivelyreferred to as sensor pixels 1510), and isolation structures 1556 a and1556 b (collectively referred to as isolation structures 1556). Theisolation structures 1556 a and 1556 b is similar to the isolationstructure 1506 described in relation to FIG. 15A, but differs in thatthe isolation structures 1556 are arranged on a portion of the substrate1502 immediately below the respective absorption regions 1504. Sucharrangement of the isolation structures 1556 between the absorptionregion 1504 and the substrate 1502 may help confine the photo-generatedcarriers to the absorption region 1504 and help reduce the leakage ofthe photo-generated carriers into the substrate 1502. For example, thesensor pixels 1510 a and 1510 b may be implemented as the switchedphotodetector 530 of FIG. 5D, which has all the electrical terminalsplaced on the absorption region 1504. In such a case, the electricalisolation provided by the isolation structure 1556 (e.g., a thin p-dopedlayer) may improve photocurrent collection efficiency and/or bandwidthof the sensor pixels 1510.

Referring to FIG. 15F, an example configuration 1560 is similar to theconfiguration 1550 of FIG. 15E, but differs in that the absorptionregions 1504 a and 1504 b are fully embedded in the substrate 1502, andthe isolation structures 1556 partially or fully surrounds theabsorption regions 1504. For isolation structures 1556 that are formedfrom insulator or dielectric, the isolation structures 1556 may includean opening below the absorber and partially surround the embeddedabsorption regions 1504. For isolation structures 1556 that are dopedregions, the isolation structures 1556 may be a continuous structurethat fully surrounds the embedded absorption regions 1504 without theopening.

While isolation structures that are doped regions, dielectric material,or insulator have been described, in general, the isolation structuremay be a combination of such implementations. Referring to FIG. 15G, anexample configuration 1570 is similar to the configuration 1500 of FIG.15A, but differ in that the isolation structure 1506 includes a firstisolation structure 1576 and a second isolation structure 1577. Thefirst isolation structure 1576 may be a trench filled with asemiconductor material that is different from the substrate 1502 or atrench filled with a dielectric or an insulator. The second isolationstructure 1577 may be a doped region that has been doped with p-typedopants or n-type dopants. The isolation structure 1504 that implementsboth different materials and doped regions may further improveelectrical isolation between the sensor pixels 1510 over isolationstructures that implement one in isolation. In some implementations, adoping isolation may be used to form the second isolation structure 1577while a material isolation through trench fill may be used to form thefirst isolation structure 1576 in which the doping isolation isshallower than the material isolation.

Light detection efficiency of a photodetector, such as a switchedphotodetector, may be enhanced by addition of various structures thatmodify optical characteristics of the photodetector. For example,mirrors, dielectric layers, and anti-reflection coating (ARC) layers canbe added alone or in combination to achieve various effects includingincreased absorption of light by an absorption region, creation of anoptical resonance cavity, and/or alteration of the spectral response ofthe photodetector. FIGS. 16A-16J show cross-sectional views of exampleconfigurations for improving detection efficiency of a photodetector.Referring to FIG. 16A, an example configuration 1600 includes asubstrate 1602, an absorption region 1604, and a metal mirror 1606. Theabsorption region 1604 forms a photodetector. The metal mirror 1606reflects incident light.

An optical signal 1605 is incident on the absorption region 1604 fromthe top as shown, which may be referred to as a front-side illumination(FSI) configuration. In such configurations, in some cases, the opticalsignal 1605 may not be fully absorbed by the absorption region 1604, anda portion of the optical signal 1605 may pass through the absorptionregion 1604. Such light that passes through the absorption region 1604without being absorbed may reduce detection efficiency of thephotodetector. By placing the metal mirror 1606 on a lower surface ofthe substrate 1602 to reflect the passed-through portion of the opticalsignal 1605, the passed-through portion may be reflected back toward theabsorption region 1604 for a second pass through the absorption region1604, improving detection efficiency.

The portion of the optical signal 1605 that gets absorbed by theabsorption region 1604 may be a function of optical absorptioncoefficient of the absorption region 1604, the thickness of theabsorption region 1604 along the direction of light incidence (e.g.,along the vertical direction), and the wavelength of the optical signal1605.

The metal mirror 1606 may be formed from various optically reflectivemetals, such as copper, aluminum, gold, and platinum. The metal mirror1606 may have reflectivity greater than 50%, 60%, 70%, 80%, 90%, or 95%at the operation wavelength of the photodetector of the configuration1600. The thickness of the metal mirror 1606 may be greater than askin-depth of the metal. For example, the metal mirror 1606 may have athickness ranging from 50 nm to 500 nm.

Referring to FIG. 16B, an example configuration 1610 is similar to theconfiguration 1600 of FIG. 16A, but differs in that the exampleconfiguration 1610 further includes a dielectric layer 1608 arrangedbetween the substrate 1602 and the metal mirror 1606. The dielectriclayer 1608 may alter an optical reflection spectrum of the metal mirror1608. For example, by thin film interference caused by the dielectriclayer 1608 (e.g., a SiO2 layer), the reflection of the light incident onthe metal mirror 1606 (e.g., an Al layer) may be enhanced (e.g., areflectivity enhanced from <90% to >97%) at certain wavelengths anddecreased at some other wavelengths.

Referring to FIG. 16C, an example configuration 1620 is similar to theconfiguration 1600 of FIG. 16A, but differs in that the metal mirror1606 of configuration 1600 has been replaced with a dielectric mirror1626. The dielectric mirror may be a single layer of dielectric film ora stack of various dielectric films. The dielectric mirror 1626 may beformed from various dielectric materials, such as SiO2, Si3N4, SiON, andSi. The dielectric mirror 1626 may have reflectivity greater than 50%,60%, 70%, 80%, 90%, or 95% at the operation wavelength of thephotodetector of the configuration 1620. The thickness of the dielectricmirror 1626 may have a thickness ranging from 50 nm to 4000 nm.

Referring to FIG. 16D, an example configuration 1630 is similar to theconfiguration 1620 of FIG. 16C, but differs in that the dielectricmirror 1626 of configuration 1620 has been replaced with a DistributedBragg Reflector (DBR) mirror 1632. The DBR mirror includes multiplefirst dielectric layers 1634 and multiple second dielectric layers 1636that are stacked on top of each other in an alternating fashion. Thesecond dielectric layers 1636 have an index of refraction that isdifferent from that of the first dielectric layers 1634. The firstlayers 1634 and the second layers 1636 may have a thickness thatcorresponds to a quarter of the operation wavelength in the respectivedielectric materials. The reflectivity and the reflection bandwidth maydepend on the thicknesses, the refractive indices of the first layers1634 and the second layers 1636, and the number of first-second layerpairs.

Referring to FIG. 16E, an example configuration 1640 includes thesubstrate 1602, the absorption region 1604, and an anti-reflectioncoating (ARC) layer 1648. The ARC layer 1648 may reduce a reflection ofthe optical signal 1605 incident on the absorption region 1604. The ARClayer 1648 may be similar to ARC layer 744 of FIG. 7B.

Referring to FIG. 16F, an example configuration 1650 is similar to theconfiguration 1600 of FIG. 16A, but differs in that the metal mirror1606 is now placed on the upper surface of the substrate 1602, on theside of the absorption region 1604. The optical signal 1605 is nowincident on the absorption region 1604 through the lower surface of thesubstrate 1602, which may be referred to as a back-side illumination(BSI) configuration. The effect of the metal mirror 1606 is analogous tothe descriptions in relation to FIG. 16A.

Referring to FIG. 16G, an example configuration 1660 is similar to theconfiguration 1610 of FIG. 16B, but differs in that the dielectric layer1608 and the metal mirror 1606 are now placed on the upper surface ofthe substrate 1602, on the side of the absorption region 1604. Theoptical signal 1605 is now incident on the absorption region 1604through the lower surface of the substrate 1602, which may be referredto as a back-side illumination (BSI) configuration. The effect of thedielectric layer 1608 and the metal mirror 1606 is analogous to thedescriptions in relation to FIG. 16B.

Referring to FIG. 16H, an example configuration 1670 is similar to theconfiguration 1620 of FIG. 16C, but differs in that the dielectricmirror 1626 is now placed on the upper surface of the substrate 1602, onthe side of the absorption region 1604. The optical signal 1605 is nowincident on the absorption region 1604 through the lower surface of thesubstrate 1602, which may be referred to as a back-side illumination(BSI) configuration. The effect of the dielectric mirror 1626 isanalogous to the descriptions in relation to FIG. 16C.

Referring to FIG. 16I, an example configuration 1680 is similar to theconfiguration 1630 of FIG. 16D, but differs in that the DBR mirror 1632is now placed on the upper surface of the substrate 1602, on the side ofthe absorption region 1604. The optical signal 1605 is now incident onthe absorption region 1604 through the lower surface of the substrate1602, which may be referred to as a back-side illumination (BSI)configuration. The effect of the DBR mirror 1632 is analogous to thedescriptions in relation to FIG. 16D.

Referring to FIG. 16J, an example configuration 1690 is similar to theconfiguration 1640 of FIG. 16E, but differs in that the ARC layer 1648is now placed on the lower surface of the substrate 1602, on the side ofthe substrate 1602 opposite to the absorption region 1604. The opticalsignal 1605 is now incident on the absorption region 1604 through thelower surface of the substrate 1602, which may be referred to as aback-side illumination (BSI) configuration. The effect of the ARC layer1648 is analogous to the descriptions in relation to FIG. 16E.

In general, the mirror structures, such as metal mirror 1606, thedielectric layer 1608, the dielectric mirror 1626, and the DBR mirror1632 may be fabricated in various ways. For example, the mirrorstructures may be deposited directly onto the substrate 1602.Alternatively, or additionally, the mirror structures may be fabricatedon a separate substrate and bonded to the substrate 1602 through waferbonding techniques.

While individual implementations having metal mirror 1606, thedielectric layer 1608, the dielectric mirror 1626, and the DBR mirror1632 on the lower surface or the upper surface of the substrate 1602 areshown, in general, the described structures may be implemented on bothsides of the substrate 1602. For example, the DBR mirror 1632 may beimplemented on both sides of the substrate 1602, which may create anoptical resonance cavity around the absorption region 1604, modifyingthe spectral response of the photodetector. As another example, the ARClayer 1648 may be implemented on the upper surface of the substrate 1602in combination with a mirror structure on the lower surface of thesubstrate 1602 (e.g., configurations 1600, 1610, 1620, and 1630) tofurther enhance detection efficiency of the photodetector. In general,mirrors such as the metal mirror 1606, the dielectric layer 1608, thedielectric mirror 1626, and the DBR mirror 1632 may be partiallyreflecting and partially transmitting.

Surface of the absorption regions may be modified in various ways tomodify various performance characteristics of a photodetector. Examplesof modification of the surface of the absorption regions include:addition of doping regions; introduction of foreign elements; variationof material composition; introduction of topographies onto the surfaceof the absorption region; and deposition of dielectric or semiconductormaterials. Examples of performance characteristics include: lightabsorption efficiency; optical absorption spectrum; carrier collectionefficiency; dark current or leakage current; photodetector operationpower; and photodetector bandwidth.

FIGS. 17A-17E show cross-sectional views of example configurations ofabsorption region surface modification. Referring to FIG. 17A, asurface-modified absorption region 1700 includes a germanium-siliconbased absorption region 1704 and a surface modification layer 1706. Thegermanium-silicon based absorption region 1704 may be an absorptionregion of a switched photodetector such as the switched photodetector530 of FIG. 5D.

The GeSi-based absorption region 1704 may be a Si_(x)Ge_(1-x) compoundwith varying composition (X). For example, the composition (X) may varyfrom 0.01, at which point the GeSi-based absorption region 1704 may havea characteristic closer to Ge, to 0.99, at which point the GeSi-basedabsorption region 1704 may be have a characteristic closer to Si. Thecomposition of the GeSi-based absorption region may affect the opticalabsorption efficiency for a given wavelength, and also affect theoverall optical absorption spectrum. For example, a composition with alower (X), corresponding to higher Ge concentration, may absorb morestrongly in the near infrared wavelengths (e.g., >1 μm) compared to acomposition with a higher (X), corresponding to a higher Si composition.

The surface modification layer 1706 may modify the optical and/orelectrical properties of the GeSi-based absorption region 1704 and thephotodetector including the absorption region 1704. The surfacemodification layer may be formed from various materials, such asamorphous silicon, polysilicon, epitaxial silicon, Si_(Y)Ge_(1-Y)compound with varying composition (Y), Ge_(Z)Sn_(1-Z) compound withvarying composition (Z), and any combination thereof.

In some implementations, for a GeSi-based absorption region 1704 havinga Si_(x)Ge_(1-x) composition, the surface modification layer 1706 may bea Si_(Y)Ge_(1-Y) layer where the compositions (X) and (Y) are different.For example, by having a composition (X) that is larger than composition(Y), the surface modification layer 1706 may have a higher absorptioncoefficient at a longer wavelength than the GeSi-based absorption region1704. As such, incident light at a longer wavelength may be stronglyabsorbed by the surface modification layer 1706 without penetrating deepinto the GeSi-based absorption region 1704. By absorbing the incidentlight closer to the surface of the GeSi-based absorption region 1704,bandwidth of the photodetector including the absorption region 1704 mayimprove due to reduced diffusion of the photo-generated carriers withinthe absorption region 1704. In some implementations, for a puregermanium absorption region 1704 (i.e., X=0), the surface modificationlayer 1706 may be a Si_(Y)Ge_(1-Y) layer. In some implementations, thecomposition of the surface modification layer 1706 and the GeSi-basedabsorption region 1704 may vary along a direction, such as the verticaldirection, forming a graded GeSi absorption region 1704. The grading ofthe GeSi composition may further improve bandwidth of the photodetector.In some implementations, the surface modification layer 1706 may bemulti-layered. For example, a GeSi layer may be deposited on top of aGeSi-based absorption region 1704 for passivation, and another Si layermay be deposited on top of the GeSi layer for further passivation.

In some implementations, the surface modification layer 1706 may be aGermanium-Tin alloy Ge_(Z)Sn_(1-Z) with varying composition (Z). Theaddition of Tin to the surface modification layer 1706 may improveoptical absorption efficiency at longer wavelengths, such as beyond thebandgap of germanium (approximately 1.55 μm), beyond which point theabsorption efficiency of pure germanium decreases significantly.

Referring to FIG. 17B, a surface-modified absorption region 1710includes the germanium-silicon based absorption region 1704 and firstdoped region 1712. In some implementations, the first doped region 1712may be doped with p-type or n-type dopants. P-type or n-type dopants maymodify the electrical properties of the absorption region 1704. Forexample, the photo-generated electrons (or holes) may be repelled awayfrom the surface due to the first doped region 1712, thereby avoidingsurface recombination, which results into a higher collection efficiencywhen first doped region 1712 is doped with p-type (or n-type) dopants.In some implementations, the first doped region 1712 may be doped withimpurities that modifies the optical property of the absorption region1704, such as silicon and tin.

Referring to FIG. 17C, a surface-modified absorption region 1720 issimilar to the surface-modified absorption region 1710, but differs inthat it further includes a second doped region 1722. The second dopedregion 1722 may be similar to the first doped region 1712 or may have adifferent polarity, depth, and width such that the photo-generatedcarriers are attracted by the second doped region 1722 and repelled bythe first doped region 1712.

Referring to FIG. 17D, a surface-modified absorption region 1730includes the germanium-silicon based absorption region 1704 anddielectric wells 1732. The dielectric wells 1732 may be filled withvarious dielectric materials, such as SiO₂, Si₃N₄, and high-k material.The dielectric well may contribute to reduction of dark current orleakage current, reduction of photodetector operation power, and/orimprovement of photodetector bandwidth, when it is placed, for example,inside a PN junction or in-between surface electrical terminals.

Referring to FIG. 17E, a switched photodetector 1740 includes a surfacemodified Ge-absorption region 1710 of FIG. 17B. The switchedphotodetector 1740 is similar to the switched photodetector 160 of FIG.1B, but differs in that it further includes the surface modificationlayer 1706, and the carrier collection terminals 1106 and carriercontrol terminals 1108 of FIG. 11A. The addition of the surfacemodification layer 1706 may contribute to improvements in variousperformance characteristics of the switched photodetector 1740, such as:light absorption efficiency; optical absorption spectrum; carriercollection efficiency; dark current or leakage current; photodetectoroperation power; and photodetector bandwidth.

While individual implementations of surface modification of theabsorption region are shown, in general, the described surfacemodification can be implemented in various combinations to achievedesired effects. For example, the surface modification layer 1706 may beimplemented in combination with the first doped region 1712 and/or thesecond doped region 1722. As another example, the surface modificationlayer 1706 may be implemented in combination with the dielectric wells1732. As yet another example, the surface modification layer 1706 may beimplemented in combination with the first doped region 1712 and/or thesecond doped region 1722, and the dielectric wells 1732.

Various doped regions and wells, such as p-doped regions and wells, andn-doped regions and wells, may be arranged in various locations of theabsorption region, the substrate, or intermediate layers to modifydevice performance characteristics. Examples of performancecharacteristics include: light absorption efficiency; optical absorptionspectrum; carrier collection efficiency; dark current or leakagecurrent; photodetector operation power; and photodetector bandwidth.

The depth of the doping regions and wells may be determined based on avariety of considerations, such as manufacturability and deviceperformance. One or more doping wells and regions may be connected to avoltage or current sources. One or more doping wells and regions may notbe connected to a voltage or current sources (i.e., floating), and/or beconnected to each other (i.e., shorted).

FIGS. 18A-18B show top and side views of an example switchedphotodetector 1800. The switched photodetector 1800 is similar to theswitched photodetector 160 of FIG. 1B, and further includes the carriercollection terminals 1106 and carrier control terminals 1108 of FIG.11A. As previous described in relation to FIG. 1B, the n-well regions152 and 154 may reduce a leakage current from the first control signal122 to the second control signal 132, and may reduce a charge couplingbetween the n-doped regions 126 and 136. Reduction of the leakagecurrent contributes to reduction of operation power of the switchedphotodetector 1800.

FIGS. 18C-18D show top and side views of an example switchedphotodetector 1820. The switched photodetector 1800 is similar to theswitched photodetector 1800 of FIGS. 18A-18B, and further includesp-well regions 1822. The p-well regions 1822 may be similar to thep-well regions 246 and 248 of FIG. 2D. The p-well regions 1822 mayincrease the collection efficiency of photo-generated electrons of theswitched photodetector 1820 relative to the switched photodetector 1800.

In some cases, the photo-generated carriers in the absorption region 106may not be completely collected by the n-doped regions 126 and 136. Insuch cases, the photo-generated carriers may reach the materialinterface between the substrate 102 and the absorption region 106, wherematerial defects may be present. The material defects may capture thephoto-generated carriers and release the carriers after some period oftime, which may be collected by n-doped regions 126 and 136. Suchcapture and release of the carriers by the material defects at theinterface and subsequent collection by the n-doped regions 126 and 136may reduce the bandwidth of the switched photodetector 1800 due to thetime delay caused by the capturing and releasing of the carriers. Assuch, such bandwidth-reduction may be mitigated by adding the p-wellregions 1822, which may block photo-generated carriers not collected bythe n-doped regions 126 and 136 from reaching the interface between theabsorption region 106 and the substrate 102.

FIG. 18E shows a top view of an example switched photodetector 1830. Theswitched photodetector 1830 is similar to the switched photodetector1820 of FIGS. 18C-18D, and further includes p-well regions 1832. Thep-well regions 1832 are similar to the p-well regions 1822. Thecombination of p-well regions 1822 and 1832 surrounds the respectiven-doped regions 126 and 136, which may further block photo-generatedcarriers not collected by the n-doped regions 126 and 136 from reachingthe interface between the absorption region 106 and the substrate 102.While shown as separate p-well regions 1822 and 1832, the p-well regions1822 and 1832 may be joined into respective “C” shaped region thatsurrounds the respective n-doped regions 126 and 136.

FIGS. 18F-18G show top and side views of an example switchedphotodetector 1840. The switched photodetector 1840 is similar to theswitched photodetector 1800 of FIGS. 18A-18B, but differs in that itomits the n-well regions 152 and 154, and includes p-well region 1842.The p-well region 1842 may be similar to the p-well regions 246 and 248of FIG. 2D. The p-well region 1842 surrounds the absorption region 106embedded within the substrate 102. The p-well region 1842 may blockphoto-generated electrons in the absorption region 106 from reaching thesubstrate 102. Such blocking may increase the collection efficiency ofphoto-generated carriers of the switched photodetector 1840 relative tothe switched photodetector 1800. The p-well region 1842 may be formed inthe absorption region 106, the substrate 102, an intermediate layerbetween the absorption region 106 and the substrate 102, or combinationthereof.

While individual implementations of n-well regions 152 and 154 andp-well regions 1822, 1832, and 1842 have been shown, in general, thedescribed n-well and p-well regions can be implemented in variouscombinations to achieve desired effects.

So far, various implementations of the elements of the switchedphotodetectors, and various arrangements of the elements have beendescribed. Now, various exemplary combinations of the previouslydescribed elements and their arrangements will be described. Thedescribed combinations are not intended to be a complete list of allcombination.

FIGS. 19A-B show top and side views of an example switched photodetector1900. The switched photodetector 1900 is similar to the switchedphotodetector 100 of FIG. 1A, but differs in that the absorption region106 of the photodetector 1900 is fully embedded in the substrate 102,and further includes the carrier collection terminals 1106 and carriercontrol terminals 1108 of FIG. 11A. The light receiving region 1205 isdescribed in relation to FIGS. 12A-12B. The presence of the p-dopedregions 128 and 138 results in formation of an Ohmic contact at theinterfaces between the carrier control terminal 1108 and the absorptionregion 106.

FIGS. 19C-D show top and side views of an example switched photodetector1910. The switched photodetector 1910 is similar to the switchedphotodetector 1900 of FIG. 19A-B, but differs in that the p-dopedregions 128 and 138 have been omitted. The omission of the p-dopedregions 128 and 138 results in formation of a Schottky junction at theinterfaces between the carrier control terminal 1108 and the absorptionregion 106.

FIGS. 19E-F show top and side views of an example switched photodetector1920. The switched photodetector 1910 is similar to the switchedphotodetector 1900 of FIG. 19A-B, but differs in that additional p-dopedregions 128 and 138, and carrier control terminals 1108 have been addedon each sides of the light receiving region 1205.

FIGS. 19G-H show top and side views of an example switched photodetector1930. The switched photodetector 1930 is similar to the switchedphotodetector 1920 of FIG. 19E-F, but differs in that the p-dopedregions 128 and 138 have been omitted. The omission of the p-dopedregions 128 and 138 results in formation of a Schottky junction at theinterfaces between the carrier control terminal 1108 and the absorptionregion 106.

FIGS. 20A-B show top and side views of an example switched photodetector2000. The switched photodetector 2000 is similar to the switchedphotodetector 1900 of FIG. 19A-B, but differs in that the intermediatelayer 1006 of FIG. 10I has been added. As previously described inrelation to FIG. 10I, the intermediate layer 1006 has an opening to thesubstrate 102, and the absorption region 106 fills the opening to thesubstrate 102 and the opening formed by the intermediate layer 1006. Insome implementations, the intermediate layer 1006 may be SiO₂, SiNx,AlOx, or any oxide or nitride-based insulators.

FIGS. 20C-D show top and side views of an example switched photodetector2010. The switched photodetector 2010 is similar to the switchedphotodetector 2000 of FIG. 19A-B, but differs in that the intermediatelayer 1006 of FIGS. 20A-B has been replaced with an intermediate layer2012. The intermediate layer 2012 is similar to the intermediate layer1006 in its material, but differs in that intermediate layer 2012 is auniform layer that extends across the upper surface of the substrate102, with openings to the substrate 102. The absorption region 106 isembedded in the opening of the intermediate layer 2012. In someimplementations, the intermediate layer 2012 may be SiO₂, SiNx, AlOx, orany oxide or nitride-based insulators.

FIGS. 20E-F show top and side views of an example switched photodetector2020. The switched photodetector 2020 is similar to the switchedphotodetector 2010 of FIGS. 20C-D, but differs in that the p-dopedregions 128 and 138 have been omitted. The omission of the p-dopedregions 128 and 138 results in formation of a Schottky junction at theinterfaces between the carrier control terminal 1108 and the absorptionregion 106.

FIGS. 20G-H show top and side views of an example switched photodetector2030. The switched photodetector 2030 is similar to the switchedphotodetector 2010 of FIGS. 20C-D, but differs in that the intermediatelayer 2012 of FIGS. 20C-D has been replaced with an intermediate layer2032. The intermediate layer 2032 is similar to the intermediate layer2012 of FIGS. 20C-D, but differs in that the intermediate layer 2032 hasa first opening 2034 to the substrate 102, and a second opening 2036that is larger than the first opening 2034 that opens up toward theupper surface of the intermediate layer 2032.

FIGS. 20I-J show top and side views of an example switched photodetector2040. The switched photodetector 2040 is similar to the switchedphotodetector 2030 of FIGS. 20G-H, but differs in that the p-dopedregions 128 and 138 have been omitted. The omission of the p-dopedregions 128 and 138 results in formation of a Schottky junction at theinterfaces between the carrier control terminal 1108 and the absorptionregion 106.

FIGS. 20K-L show top and side views of an example switched photodetector2050. The switched photodetector 2050 is similar to the switchedphotodetector 2030 of FIGS. 20G-H, but differs in that the n-wellregions 152 and 154 have been added. The n-well regions 152 and 154 havebeen described in relation to FIG. 1B.

FIGS. 21A-B show top and side views of an example switched photodetector2100. The switched photodetector 2100 is similar to the switchedphotodetector 1900 of FIGS. 19A-B, but differs in that the n-dopedregions 126 and 136, the p-doped regions 128 and 138, the carriercollection terminals 1106, and the carrier control terminals 1108 havebeen moved from the absorption region 106 to the substrate 102. Suchterminals 1106 and 1108 may be referred to as substrate carriercollection terminals and substrate carrier control terminals.

FIGS. 21C-D show top and side views of an example switched photodetector2110. The switched photodetector 2110 is similar to the switchedphotodetector 2100 of FIGS. 21A-B, but differs in that absorber p-dopedregions 2128 and 2138, and absorber carrier control terminals 2108 havebeen placed on the absorption region 106. The substrate carriercollection terminals 1106, the substrate carrier control terminals 1108,and the absorber carrier control terminals 2108 may be similar to thesubstrate carrier collection terminal 1306, the substrate carriercontrol terminal 1308, and the absorber carrier control terminal 1309described in relation to FIG. 14A, and have similar effects.

FIGS. 21E-F show top and side views of an example switched photodetector2120. The switched photodetector 2120 is similar to the switchedphotodetector 2110 of FIGS. 21C-D, but differs in that the absorberp-doped regions 2128 and 2138 have been omitted. The omission of theabsorber p-doped regions 2128 and 2138 results in formation of aSchottky junction at the interfaces between the absorber carrier controlterminal 2108 and the absorption region 106.

FIGS. 22A-B show top and side views of an example switched photodetector2200. The switched photodetector 2200 is similar to the switchedphotodetector 1840 of FIGS. 18F-G, but differs in that the n-wellregions 152 and 154 of FIGS. 18A-B have been added.

FIGS. 22C-D show top and side views of an example switched photodetector2210. The switched photodetector 2210 is similar to the switchedphotodetector 2110 of FIGS. 21C-D, but differs in that the n-wellregions 152 and 154 of FIGS. 18A-B have been added.

FIG. 23A show a top view of an example switched photodetector 2300, andFIG. 23B shows a side view of the example switched photodetector 2300along a line AA. The switched photodetector 2300 is similar to theswitched photodetector 2110 of FIGS. 21C-D, but differs in that thep-well regions 2302 have been added at the interface between theabsorption region 106 and the substrate 102. The p-well regions 2302 mayhelp mitigate carrier trapping and releasing at the interface betweenthe absorption region 106 and the substrate 102, which has beendescribed in relation to FIG. 18C-D.

FIGS. 24A-B show top and side views of an example switched photodetector2400. The switched photodetector 2400 is similar to the switchedphotodetector 1820 of FIGS. 18C-D, but differs in that the n-wellregions 152 and 154 of have been omitted.

FIG. 24C shows a top view of an example switched photodetector 2410. Theswitched photodetector 2410 is similar to the switched photodetector1830 of FIG. 18E, but differs in that the p-well regions 1822 and 1832of FIG. 18E have been merged into continuous p-well regions 2412.

FIGS. 24D-E show top and side views of an example switched photodetector2420. The switched photodetector 2420 is similar to the switchedphotodetector 2400 of FIGS. 24A-B, but differs in that dielectric wells2422 have been added in the n-doped regions 126 and 136. The dielectricwells 2422 is similar to the dielectric wells 1732 of FIG. 17D. Thedielectric well 2422 are arranged in a portion of the n-doped region 126between carrier collection terminal 1106 and the carrier controlterminal 1108. The dielectric well 2422 may reduce a dark currentbetween the carrier collection terminal 1106 and the carrier controlterminal 1108. The depth of the dielectric well 2422 may be less than,equal to, or greater than the depth of the n-doped region 126.

FIGS. 24F-G show top and side views of an example switched photodetector2430. The switched photodetector 2430 is similar to the switchedphotodetector 2420 of FIGS. 24D-E, but differs in that dielectric wells2422 have been moved from the n-doped regions 126 and 136 to the p-dopedregions 128 and 138. The depth of the dielectric well 2422 may be lessthan, equal to, or greater than the depth of the p-doped region 128. Ingeneral, the dielectric well 2422 may be placed anywhere in between then-doped region 126 and the p-doped region 128, and between the n-dopedregion 136 and the p-doped region 138.

FIGS. 25A-B show top and side views of an example switched photodetector2500. The switched photodetector 2500 is similar to the switchedphotodetector 1900 of FIGS. 19A-B, but differs in that the metal mirror1606 of FIG. 16F has been added as a metal mirror 2502 on an uppersurface of the absorption region 106, the surface on which the carriercollection terminals 1106 and carrier control terminals 1108 arelocated. The metal mirror 2502 may be placed above the light receivingregion 1205. In some implementations, the metal mirror 2502 may beimplemented by the first metal layer (M1) or the second metal layer (M2)process in CMOS fabrication or a combination of thereof.

FIGS. 25C-D show top and side views of an example switched photodetector2510. The switched photodetector 2510 is similar to the switchedphotodetector 2500 of FIGS. 25A-B, but differs in that the p-dopedregions 128 and 138 have been omitted. The omission of the p-dopedregions 128 and 138 results in formation of a Schottky junction at theinterfaces between the carrier control terminal 1108 and the absorptionregion 106.

FIGS. 25E-F show top and side views of an example switched photodetector2520. The switched photodetector 2520 is similar to the switchedphotodetector 2050 of FIGS. 20K-L, but differs in that the metal mirror1606 of FIG. 16F has been added as a metal mirror 2502 on an uppersurface of the absorption region 106, the surface on which the carriercollection terminals 1106 and carrier control terminals 1108 arelocated. The metal mirror 2502 may be placed above the light receivingregion 1205. In some implementations, the metal mirror 2502 may beimplemented by the first metal layer (M1) or the second metal layer (M2)process in CMOS fabrication or a combination of thereof.

FIGS. 25G-H show top and side views of an example switched photodetector2530. The switched photodetector 2530 is similar to the switchedphotodetector 1840 of FIGS. 18F-G, but differs in that the metal mirror1606 of FIG. 16F has been added as a metal mirror 2502 on an uppersurface of the absorption region 106, the surface on which the carriercollection terminals 1106 and carrier control terminals 1108 arelocated. The metal mirror 2502 may be placed above the light receivingregion 1205. In some implementations, the metal mirror 2502 may beimplemented by the first metal layer (M1) or the second metal layer (M2)process in CMOS fabrication or a combination of thereof.

In typical implementations of an image sensor, multiple sensor pixels(e.g., switched photodetectors) are arranged in an array to allow theimage sensor to capture images having multiple image pixels.Square-shaped sensor pixels having equal dimensions on the two sideswhen seen from the top allows for simple 2D array. However, for certainapplications such as ToF, some sensor pixels may have non-square shapes,such as a rectangular shape. For example, referring back to FIG. 1B, theswitched photodetector 160 has two carrier control terminals (e.g.,p-doped regions 128 and 138) and two carrier collection terminals (e.g.,n-doped regions 126 and 136). These four terminals are typicallyarranged along a line, which leads to a rectangular sensor pixel shapethat is longer along the line on which the terminals line up (e.g.,switched photodetector 1800 of FIG. 18A).

Such rectangular sensor pixels may present challenges in efficientarraying of the pixels due to, for example, design rules associated withsemiconductor fabrication in a foundry. Design rules may impose variousminimum separations of features such as doped regions, doped wells,dielectric wells, and germanium absorption regions. One approach toimproving compactness and symmetry is by creating a unit cell ofphotodetectors that include four rectangular photodetectors. FIG. 26show an example unit cell of rectangular photodetectors. A unit cell2600 includes four switched photodetectors 1800 of FIG. 18A and fourisolation structures 2602 that surround each of the switchedphotodetectors 1800. The isolation structures 2602 have been describedin relation to FIGS. 15A-D. The unit cell 2600 may improve sensor pixelcompactness and symmetry over the rectangular unit cell.

FIG. 27 shows a top view of an example rectangular switchedphotodetector 2700 with photo-transistor gain. The switchedphotodetector 2700 is similar to the switched photodetector 1800 of FIG.18A, but differs in that electron emitters 2710 have been added on tothe substrate 102. The electron emitter 2710 may be similar to then-doped regions 126 and 136. The rectangular shape of the switchedphotodetector 1800 allows coupling of a photocurrent integrationcapacitor (e.g., a floating diffusion capacitor) to a bipolar junctiontransistor (BJT) 2720 formed by the n-doped regions 126 and 136, thep-doped regions 128 and 138, and the electron emitter 2710, resulting inan NPN BJT. The BJT 2720, when biased appropriately, may provide aphoto-transistor gain in response to incident optical signal, which mayimprove a light to photocurrent conversion efficiency of thephotodetector 2700. For example, the BJT 2720 may be biased as follows:bias the n-doped regions 126 and 136 between 1V and 3V, bias the p-dopedregions 128 and 138 between 0V and 1V, and bias the electron emitter2710 to be lower than the bias of the respective n-doped regions 126 and136.

In general, the electron emitter 2710 and/or the n-doped regions 126 and136 should be biased to an external voltage or be shorted with a p-dopedregion through a metal connection to allow electrons to be emitted bythe electron emitter 2710.

While various implementations of switched photodetectors with aparticular combination and arrangement of n-type and p-type regions andwells have been described, in general, the polarity of the doped regionsand wells may be reversed and achieve analogous operation andfunctionality. For example, all instances of a p-well and p-dopedregions may be converted to n-well and n-doped regions, respectively,and all instance of n-well and n-doped regions may be converted top-well and p-doped regions, receptively.

FIG. 28A shows an example imaging system 2800 for determiningcharacteristics of a target object 2810. The target object 2810 may be athree-dimensional object. The imaging system 2800 may include atransmitter unit 2802, a receiver unit 2804, and a processing unit 2806.In general, the transmitter unit 2802 emits light 2812 towards thetarget object 2810. The transmitter unit 2802 may include one or morelight sources, control circuitry, and/or optical elements. For example,the transmitter unit 2802 may include one or more NIR LEDs or lasers,where the emitted light 2812 may be collimated by a collimating lens topropagate in free space.

In general, the receiver unit 2804 receives the reflected light 2814that is reflected from the target object 2810. The receiver unit 2804may include one or more photodetectors, control circuitry, and/oroptical elements. For example, the receiver unit 2804 may include animage sensor, where the image sensor includes multiple pixels fabricatedon a semiconductor substrate. Each pixel may include one or moreswitched photodetectors for detecting the reflected light 2814, wherethe reflected light 2814 may be focused to the switched photodetectors.Each switched photodetector may be a switched photodetector disclosed inthis application.

In general, the processing unit 2806 processes the photo-carriersgenerated by the receiver unit 2804 and determines characteristics ofthe target object 2810. The processing unit 2806 may include controlcircuitry, one or more processors, and/or computer storage medium thatmay store instructions for determining the characteristics of the targetobject 2810. For example, the processing unit 2806 may include readoutcircuits and processors that can process information associated with thecollected photo-carriers to determine the characteristics of the targetobject 2810. In some implementations, the characteristics of the targetobject 2810 may be depth information of the target object 2810. In someimplementations, the characteristics of the target object 2810 may bematerial compositions of the target object 2810.

FIG. 28B shows one example technique for determining characteristics ofthe target object 2810. The transmitter unit 2802 may emit light pulses2812 modulated at a frequency f_(m) with a duty cycle of 50%. Thereceiver unit 2804 may receive reflected light pulses 2814 having aphase shift of Φ. The switched photodetectors are controlled such thatthe readout circuit 1 reads the collected charges Q₁ in a phasesynchronized with the emitted light pulses, and the readout circuit 2reads the collected charges Q₂ in an opposite phase with the emittedlight pulses. In some implementations, the distance, D, between theimaging system 2800 and the target object 2810 may be derived using theequation

$\begin{matrix}{{D = {\frac{c}{4f_{m}}\frac{Q_{2}}{Q_{1} + Q_{2}}}},} & (1)\end{matrix}$

where c is the speed of light.

FIG. 28C shows another example technique for determining characteristicsof the target object 2810. The transmitter unit 2802 may emit lightpulses 2812 modulated at a frequency f_(m) with a duty cycle of lessthan 50%. By reducing the duty cycle of the optical pulses by a factorof N, but increasing the intensity of the optical pulses by a factor ofN at the same time, the signal-to-noise ratio of the received reflectedlight pulses 2814 may be improved while maintaining substantially thesame power consumption for the imaging system 2800. This is madepossible when the device bandwidth is increased so that the duty cycleof the optical pulses can be decreased without distorting the pulseshape. The receiver unit 2804 may receive reflected light pulses 2814having a phase shift of Φ. The multi-gate photodetectors are controlledsuch that a readout circuit 1 reads the collected charges Q₁′ in a phasesynchronized with the emitted light pulses, and a readout circuit 2reads the collected charges Q₂′ in a delayed phase with the emittedlight pulses. In some implementations, the distance, D, between theimaging system 2800 and the target object 2810 may be derived using theequation

$\begin{matrix}{D = {\frac{c}{4{Nf}_{m}}{\frac{Q_{2}^{\prime}}{Q_{1}^{\prime} + Q_{2}^{\prime}}.}}} & (2)\end{matrix}$

FIG. 29 shows an example of a flow diagram 2900 for determiningcharacteristics of an object using an imaging system. The process 2900may be performed by a system such as the imaging system 2800.

The system receives reflected light (2902). For example, the transmitterunit 2802 may emit NIR light pulses 2812 towards the target object 2810.The receiver unit 2804 may receive the reflected NIR light pulses 2814that is reflected from the target object 2810.

The system determines phase information (2904). For example, thereceiver unit 2804 may include an image sensor, where the image sensorincludes multiple pixels fabricated on a semiconductor substrate. Eachpixel may include one or more switched photodetectors for detecting thereflected light pulses 2814. The type of switched photodetectors may bea switched photodetector disclosed in this application, where the phaseinformation may be determined using techniques described in reference toFIG. 28B or FIG. 28C.

The system determines object characteristics (2906). For example, theprocessing unit 2806 may determine depth information of the object 2810based on the phase information using techniques described in referenceto FIG. 28B or FIG. 28C.

In some implementations, an image sensor includes multiple pixels arefabricated on a semiconductor substrate, where each pixel may includeone or more switched photodetectors 100, 160, 170, 180, 200, 250, 260,270, 300, 360, 370, 380, 400, 450, 460, 470, and 480 for detecting thereflected light as illustrated in FIGS. 28A and 28B. The isolationbetween these pixels may be implemented based on an insulator isolationsuch as using an oxide or nitride layer, or based on an implantisolation such as using p-type or n-type region to block signalelectrons or holes, or based on an intrinsic built-in energy barriersuch as a using the germanium-silicon heterojunction interface.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the disclosure. For example, various formsof the flows shown above may be used, with steps re-ordered, added, orremoved.

Various implementations may have been discussed using two-dimensionalcross-sections for easy description and illustration purpose.Nevertheless, the three-dimensional variations and derivations shouldalso be included within the scope of the disclosure as long as there arecorresponding two-dimensional cross-sections in the three-dimensionalstructures.

While this specification contains many specifics, these should not beconstrued as limitations, but rather as descriptions of featuresspecific to particular embodiments. Certain features that are describedin this specification in the context of separate embodiments may also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment mayalso be implemented in multiple embodiments separately or in anysuitable subcombination. Moreover, although features may be describedabove as acting in certain combinations and even initially claimed assuch, one or more features from a claimed combination may in some casesbe excised from the combination, and the claimed combination may bedirected to a subcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described program components and systems maygenerally be integrated together in a single software product orpackaged into multiple software products.

Thus, particular embodiments have been described. Other embodiments arewithin the scope of the following claims. For example, the actionsrecited in the claims may be performed in a different order and stillachieve desirable results.

What is claimed is:
 1. An optical apparatus comprising: a semiconductorsubstrate; a first light absorption region supported by thesemiconductor substrate, the first light absorption region configured toabsorb photons and to generate photo-carriers from the absorbed photons;one or more first switches controlled by a first control signal, the oneor more first switches configured to collect at least a portion of thephoto-carriers based on the first control signal; and one or more secondswitches controlled by a second control signal, the one or more secondswitches configured to collect at least a portion of the photo-carriersbased on the second control signal, wherein the second control signal isdifferent from the first control signal, wherein the one or more firstswitches comprise: a first p-doped region in the first light absorptionregion, wherein the first p-doped region is controlled by the firstcontrol signal and has a first p-dopant concentration; a second p-dopedregion in the first light absorption region and in contact with at leasta first portion of the first p-doped region, wherein the second p-dopedregion has a second p-dopant concentration lower than the first p-dopantconcentration; and a first n-doped region in the first light absorptionregion, wherein the first n-doped region is coupled to a first readoutintegrated circuit and has a first n-dopant concentration, wherein theone or more second switches comprise: a third p-doped region in thefirst light absorption region, wherein the third p-doped region iscontrolled by the second control signal and has a third p-dopantconcentration; a fourth p-doped region in the first light absorptionregion and in contact with at least a first portion of the third p-dopedregion, wherein the fourth p-doped region has a fourth p-dopantconcentration lower than the third p-dopant concentration; and a secondn-doped region in the first light absorption region, wherein the secondn-doped region is coupled to a second readout integrated circuit and hasa second n-dopant concentration.
 2. The optical apparatus of claim 1,wherein during operation, the second p-doped region reduces a first darkcurrent flowing between the first p-doped region and the first n-dopedregion, and the fourth p-doped region reduces a second dark currentflowing between the third p-doped region and the second n-doped regionrelative to a comparable optical apparatus without the second and fourthp-doped regions.
 3. The optical apparatus of claim 1, wherein the one ormore first switches further comprise: a third n-doped region in thefirst light absorption region and in contact with at least a portion ofthe first n-doped region, wherein the third n-doped region has a thirdn-dopant concentration lower than the first n-dopant concentration, andwherein the one or more second switches further comprise: a fourthn-doped region in the first light absorption region and in contact withat least a portion of the second n-doped region, wherein the fourthn-doped region has a fourth n-dopant concentration lower than the secondn-dopant concentration.
 4. The optical apparatus of claim 3, whereinduring operation, the third n-doped region reduces a first dark currentflowing between the first p-doped region and the first n-doped region,and the fourth n-doped region reduces a second dark current flowingbetween the third p-doped region and the second n-doped region relativeto a comparable optical apparatus without the third and fourth n-dopedregions.
 5. The optical apparatus of claim 1, wherein the first lightabsorption region comprises germanium or germanium-silicon.
 6. Theoptical apparatus of claim 5, further comprising: a first layersupported by the first light absorption region, the first layer beingdifferent from the first light absorption region.
 7. The opticalapparatus of claim 6, wherein the one or more first switches furthercomprise: a fifth n-doped region in contact with a second portion of thefirst p-doped region, and wherein the one or more second switchesfurther comprise: a sixth n-doped region in contact with a secondportion the third p-doped region.
 8. An optical apparatus comprising: asemiconductor substrate; a first light absorption region supported bythe semiconductor substrate, the first light absorption regionconfigured to absorb photons and to generate photo-carriers from theabsorbed photons; one or more first switches controlled by a firstcontrol signal, the one or more first switches configured to collect atleast a portion of the photo-carriers based on the first control signal;and one or more second switches controlled by a second control signal,the one or more second switches configured to collect at least a portionof the photo-carriers based on the second control signal, wherein thesecond control signal is different from the first control signal,wherein the one or more first switches comprise: a first p-doped regionin the first light absorption region, wherein the first p-doped regionis controlled by the first control signal and has a first p-dopantconcentration; a first n-doped region in the first light absorptionregion, wherein the first n-doped region is coupled to a first readoutintegrated circuit and has a first n-dopant concentration; and a firsttrench located between the first p-doped region and the first n-dopedregion, wherein the one or more second switches comprise: a secondp-doped region in the first light absorption region, wherein the secondp-doped region is controlled by the second control signal and has asecond p-dopant concentration; a second n-doped region in the firstlight absorption region, wherein the second n-doped region is coupled toa second readout integrated circuit and has a second n-dopantconcentration; and a second trench located between the second p-dopedregion and the second n-doped region.
 9. The optical apparatus of claim8, wherein during operation, the first trench reduces a first darkcurrent flowing between the first p-doped region and the first n-dopedregion, and the second trench reduces a second dark current flowingbetween the second p-doped region and the second n-doped region relativeto a comparable optical apparatus without the first and second trenches.10. The optical apparatus of claim 8, wherein the first light absorptionregion comprises germanium or germanium-silicon.
 11. The opticalapparatus of claim 8, wherein the one or more first switches furthercomprise: a third p-doped region in the first light absorption regionand in contact with at least a first portion of the first p-dopedregion, wherein the third p-doped region has a third p-dopantconcentration lower than the first p-dopant concentration; and a thirdn-doped region in the first light absorption region and in contact withat least a portion of the first n-doped region, wherein the thirdn-doped region has a third n-dopant concentration lower than the firstn-dopant concentration, and wherein the one or more second switchesfurther comprise: a fourth p-doped region in the first light absorptionregion and in contact with at least a first portion of the secondp-doped region, wherein the fourth p-doped region has a fourth p-dopantconcentration lower than the second p-dopant concentration; and a fourthn-doped region in the first light absorption region and in contact withat least a portion of the second n-doped region, wherein the fourthn-doped region has a fourth n-dopant concentration lower than the secondn-dopant concentration.
 12. The optical apparatus of claim 11, whereinduring operation, the third n-doped region and the third p-doped regionreduces a first dark current flowing between the first p-doped regionand the first n-doped region, and the fourth n-doped region and thefourth p-doped region reduces a second dark current flowing between thesecond p-doped region and the second n-doped region relative to acomparable optical apparatus without the third and fourth n-dopedregions and the third and fourth p-doped regions.
 13. The opticalapparatus of claim 10, further comprising: a first layer supported bythe first light absorption region, the first layer being different fromthe first light absorption region and covering the first trench and thesecond trench.
 14. The optical apparatus of claim 13, wherein the one ormore first switches further comprise: a fifth n-doped region in contactwith a second portion of the first p-doped region; and wherein the oneor more second switches further comprise: a sixth n-doped region incontact with a second portion the second p-doped region.
 15. The opticalapparatus of claim 8, wherein the first trench and the second trench areat least partially filled with a dielectric material.